In vitro networks: cortical mechanisms of anaesthetic action

Deficient synaptic neurotransmission results in a persistent sleep-like cortical activity across vigilance states in mice

Growing evidence suggests that brain activity during sleep, as well as sleep regulation, are tightly linked with synaptic function and network excitability at the local and global levels. We previously reported that a mutation in synaptobrevin 2 (Vamp2) in restless (rlss) mice results in a marked increase of wakefulness and suppression of sleep, in particular REM sleep (REMS), as well as increased consolidation of sleep and wakefulness. In this study, using finer-scale in vivo electrophysiology recordings, we report that spontaneous cortical activity in rlss mice during NREM sleep (NREMS) is characterized by an occurrence of abnormally prolonged periods of complete neuronal silence (OFF-periods), often lasting several seconds, similar to the burst suppression pattern typically seen under deep anesthesia. Increased incidence of prolonged network OFF-periods was not specific to NREMS but also present in REMS and wake in rlss mice. Slow-wave activity (SWA) was generally increased in rlss mice relative to controls, while higher frequencies, including theta-frequency activity, were decreased, further resulting in diminished differences between vigilance states. The relative increase in SWA after sleep deprivation was attenuated in rlss mice, suggesting either that rlss mice experience persistently elevated sleep pressure or, alternatively, that the intrusion of sleep-like patterns of activity into the wake state attenuates the accumulation of sleep drive. We propose that a deficit in global synaptic neurotransmitter release leads to "state inertia," reflected in an abnormal propensity of brain networks to enter and remain in a persistent "default state" resembling coma or deep anesthesia.

Changes in the Term Neonatal Electroencephalogram with General Anesthesia: A Systematic Review with Narrative Synthesis

BACKGROUND

Although effects of general anesthesia on neuronal activity in the human neonatal brain are incompletely understood, electroencephalography provides some insight and may identify age-dependent differences.

METHODS

A systematic search (MEDLINE, Embase, PubMed, and Cochrane Library to November 2023) retrieved English language publications reporting electroencephalography during general anesthesia for cardiac or noncardiac surgery in term neonates (37 to 44 weeks postmenstrual age). Data were extracted, and risk of bias (ROBINS-I Cochrane tool) and quality of evidence (Grading of Recommendations Assessment, Development, and Evaluation [GRADE] checklist) were assessed.

RESULTS

From 1,155 abstracts, 9 publications (140 neonates; 55% male) fulfilled eligibility criteria. Data were limited, and study quality was very low. The occurrence of discontinuity, a characteristic pattern of alternating higher and lower amplitude electroencephalography segments, was reported with general anesthesia (94 of 119 neonates, 6 publications) and with hypothermia (23 of 23 neonates, 2 publications). Decreased power in the delta (0.5 to 4 Hz) frequency range was also reported with increasing anesthetic dose (22 neonates; 3 publications).

CONCLUSION

Although evidence gaps were identified, both increasing sevoflurane concentration and decreasing temperature are associated with increasing discontinuity.

EDITOR’S PERSPECTIVE

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.

In silico voltage-sensitive dye imaging reveals the emergent dynamics of cortical populations

Voltage-sensitive dye imaging (VSDI) is a powerful technique for interrogating membrane potential dynamics in assemblies of cortical neurons, but with effective resolution limits that confound interpretation. To address this limitation, we developed an in silico model of VSDI in a biologically faithful digital reconstruction of rodent neocortical microcircuitry. Using this model, we extend previous experimental observations regarding the cellular origins of VSDI, finding that the signal is driven primarily by neurons in layers 2/3 and 5, and that VSDI measurements do not capture individual spikes. Furthermore, we test the capacity of VSD image sequences to discriminate between afferent thalamic inputs at various spatial locations to estimate a lower bound on the functional resolution of VSDI. Our approach underscores the power of a bottom-up computational approach for relating scales of cortical processing.

Inferring a simple mechanism for alpha-blocking by fitting a neural population model to EEG spectra

Alpha blocking, a phenomenon where the alpha rhythm is reduced by attention to a visual, auditory, tactile or cognitive stimulus, is one of the most prominent features of human electroencephalography (EEG) signals. Here we identify a simple physiological mechanism by which opening of the eyes causes attenuation of the alpha rhythm. We fit a neural population model to EEG spectra from 82 subjects, each showing a different degree of alpha blocking upon opening of their eyes. Though it has been notoriously difficult to estimate parameters by fitting such models, we show how, by regularizing the differences in parameter estimates between eyes-closed and eyes-open states, we can reduce the uncertainties in these differences without significantly compromising fit quality. From this emerges a parsimonious explanation for the spectral differences between states: Changes to just a single parameter, p_ei, corresponding to the strength of a tonic excitatory input to the inhibitory cortical population, are sufficient to explain the reduction in alpha rhythm upon opening of the eyes. We detect this by comparing the shift in each model parameter between eyes-closed and eyes-open states. Whereas changes in most parameters are weak or negligible and do not scale with the degree of alpha attenuation across subjects, the change in p_ei increases monotonically with the degree of alpha blocking observed. These results indicate that opening of the eyes reduces alpha activity by increasing external input to the inhibitory cortical population.

One of the most striking features of the human electroencephalogram (EEG) is the presence of neural oscillations in the range of 8-13 Hz. It is well known that attenuation of these alpha oscillations, a process known as alpha blocking, arises from opening of the eyes, though the cause has remained obscure. In this study we infer the mechanism underlying alpha blocking by fitting a neural population model to EEG spectra from 82 different individuals. Although such models have long held the promise of being able to relate macroscopic recordings of brain activity to microscopic neural parameters, their utility has been limited by the difficulty of inferring these parameters from fits to data. Our approach involves fitting eyes-open and eyes-closed EEG spectra in a way that minimizes unnecessary differences in model parameters between the two states. Surprisingly, we find that changes in just one parameter, the level of external input to the inhibitory neurons in cortex, is sufficient to explain the attenuation of alpha oscillations. This indicates that opening of the eyes reduces alpha activity simply by increasing external inputs to the inhibitory neurons in the cortex.

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.

Understanding the Effects of General Anesthetics on Cortical Network Activity Using Ex Vivo Preparations

General anesthetics have been used to ablate consciousness during surgery for more than 150 yr. Despite significant advances in our understanding of their molecular-level pharmacologic effects, comparatively little is known about how anesthetics alter brain dynamics to cause unconsciousness. Consequently, while anesthesia practice is now routine and safe, there are many vagaries that remain unexplained. In this paper, the authors review the evidence that cortical network activity is particularly sensitive to general anesthetics, and suggest that disruption to communication in, and/or among, cortical brain regions is a common mechanism of anesthesia that ultimately produces loss of consciousness. The authors review data from acute brain slices and organotypic cultures showing that anesthetics with differing molecular mechanisms of action share in common the ability to impair neurophysiologic communication. While many questions remain, together, ex vivo and in vivo investigations suggest that a unified understanding of both clinical anesthesia and the neural basis of consciousness is attainable.

Differences in Motor Evoked Potentials Induced in Rats by Transcranial Magnetic Stimulation under Two Separate Anesthetics: Implications for Plasticity Studies

Repetitive transcranial magnetic stimulation (rTMS) is primarily used in humans to change the state of corticospinal excitability. To assess the efficacy of different rTMS stimulation protocols, motor evoked potentials (MEPs) are used as a readout due to their non-invasive nature. Stimulation of the motor cortex produces a response in a targeted muscle, and the amplitude of this twitch provides an indirect measure of the current state of the cortex. When applied to the motor cortex, rTMS can alter MEP amplitude, however, results are variable between participants and across studies. In addition, the mechanisms underlying any change and its locus are poorly understood. In order to better understand these effects, MEPs have been investigated in vivo in animal models, primarily in rats. One major difference in protocols between rats and humans is the use of general anesthesia in animal experiments. Anesthetics are known to affect plasticity-like mechanisms and so may contaminate the effects of an rTMS protocol. In the present study, we explored the effect of anesthetic on MEP amplitude, recorded before and after intermittent theta burst stimulation (iTBS), a patterned rTMS protocol with reported facilitatory effects. MEPs were assessed in the brachioradialis muscle of the upper forelimb under two anesthetics: a xylazine/zoletil combination and urethane. We found MEPs could be induced under both anesthetics, with no differences in the resting motor threshold or the average baseline amplitudes. However, MEPs were highly variable between animals under both anesthetics, with the xylazine/zoletil combination showing higher variability and most prominently a rise in amplitude across the baseline recording period. Interestingly, application of iTBS did not facilitate MEP amplitude under either anesthetic condition. Although it is important to underpin human application of TMS with mechanistic examination of effects in animals, caution must be taken when selecting an anesthetic and in interpreting results during prolonged TMS recording.

HCN1 Channels Contribute to the Effects of Amnesia and Hypnosis but not Immobility of Volatile Anesthetics

BACKGROUND

Hyperpolarization-activated, cyclic nucleotide-gated (HCN) subtype 1 (HCN1) channels have been identified as targets of ketamine to produce hypnosis. Volatile anesthetics also inhibit HCN1 channels. However, the effects of HCN1 channels on volatile anesthetics in vivo are still elusive. This study uses global and conditional HCN1 knockout mice to evaluate how HCN1 channels affect the actions of volatile anesthetics.

METHODS

Minimum alveolar concentrations (MACs) of isoflurane and sevoflurane that induced immobility (MAC of immobility) and/or hypnosis (MAC of hypnosis) were determined in wild-type mice, global HCN1 knockout (HCN1) mice, HCN1 channel gene with 2 lox-P sites flanking a region of the fourth exon of HCN1 (HCN1) mice, and forebrain-selective HCN1 knockout (HCN1: cre) mice. Immobility of mice was defined as no purposeful reactions to tail-clamping stimulus, and hypnosis was defined as loss of righting reflex. The amnestic effects of isoflurane and sevoflurane were evaluated by fear-potentiated startle in these 4 strains of mice.

RESULTS

All MAC values were expressed as mean ± SEM. For MAC of immobility of isoflurane, no significant difference was found among wild-type, HCN1, HCN1, and HCN1: cre mice (all ~1.24%-1.29% isoflurane). For both HCN1 and HCN1: cre mice, the MAC of hypnosis for isoflurane (each ~1.05% isoflurane) was significantly increased over their nonknockout controls: HCN1 versus wild-type (0.86% ± 0.03%, P < 0.001) and HCN1: cre versus HCN1 mice (0.84% ± 0.03%, P < 0.001); no significant difference was found between HCN1 and HCN1: cre mice. For MAC of immobility of sevoflurane, no significant difference was found among wild-type, HCN1, HCN1, and HCN1: cre mice (all ~2.6%-2.7% sevoflurane). For both HCN1 and HCN1: cre mice, the MAC of hypnosis for sevoflurane (each ~1.90% sevoflurane) was significantly increased over their nonknockout controls: HCN1 versus wild-type (1.58% ± 0.05%, P < 0.001) and HCN1: cre versus HCN1 mice (1.56% ± 0.05%, P < 0.001). No significant difference was found between HCN1 and HCN1: cre mice. By fear-potentiated startle experiments, amnestic effects of isoflurane and sevoflurane were significantly attenuated in HCN1 and HCN1: cre mice (both P < 0.002 versus wild-type or HCN1 mice). No significant difference was found between HCN1 and HCN1: cre mice.

CONCLUSIONS

Forebrain HCN1 channels contribute to hypnotic and amnestic effects of volatile anesthetics, but HCN1 channels are not involved in the immobilizing actions of volatile anesthetics.

How the cortico-thalamic feedback affects the EEG power spectrum over frontal and occipital regions during propofol-induced sedation

Increasing concentrations of the anaesthetic agent propofol initially induces sedation before achieving full general anaesthesia. During this state of anaesthesia, the observed specific changes in electroencephalographic (EEG) rhythms comprise increased activity in the δ- (0.5-4 Hz) and α- (8-13 Hz) frequency bands over the frontal region, but increased δ- and decreased α-activity over the occipital region. It is known that the cortex, the thalamus, and the thalamo-cortical feedback loop contribute to some degree to the propofol-induced changes in the EEG power spectrum. However the precise role of each structure to the dynamics of the EEG is unknown. In this paper we apply a thalamo-cortical neuronal population model to reproduce the power spectrum changes in EEG during propofol-induced anaesthesia sedation. The model reproduces the power spectrum features observed experimentally both in frontal and occipital electrodes. Moreover, a detailed analysis of the model indicates the importance of multiple resting states in brain activity. The work suggests that the α-activity originates from the cortico-thalamic relay interaction, whereas the emergence of δ-activity results from the full cortico-reticular-relay-cortical feedback loop with a prominent enforced thalamic reticular-relay interaction. This model suggests an important role for synaptic GABAergic receptors at relay neurons and, more generally, for the thalamus in the generation of both the δ- and the α- EEG patterns that are seen during propofol anaesthesia sedation.

Study of GABAergic extra-synaptic tonic inhibition in single neurons and neural populations by traversing neural scales: application to propofol-induced anaesthesia

Anaesthetic agents are known to affect extra-synaptic GABAergic receptors, which induce tonic inhibitory currents. Since these receptors are very sensitive to small concentrations of agents, they are supposed to play an important role in the underlying neural mechanism of general anaesthesia. Moreover anaesthetic agents modulate the encephalographic activity (EEG) of subjects and hence show an effect on neural populations. To understand better the tonic inhibition effect in single neurons on neural populations and hence how it affects the EEG, the work considers single neurons and neural populations in a steady-state and studies numerically and analytically the modulation of their firing rate and nonlinear gain with respect to different levels of tonic inhibition. We consider populations of both type-I (Leaky Integrate-and-Fire model) and type-II (Morris-Lecar model) neurons. To bridge the single neuron description to the population description analytically, a recently proposed statistical approach is employed which allows to derive new analytical expressions for the population firing rate for type-I neurons. In addition, the work shows the derivation of a novel transfer function for type-I neurons as considered in neural mass models and studies briefly the interaction of synaptic and extra-synaptic inhibition. We reveal a strong subtractive and divisive effect of tonic inhibition in type-I neurons, i.e. a shift of the firing rate to higher excitation levels accompanied by a change of the nonlinear gain. Tonic inhibition shortens the excitation window of type-II neurons and their populations while maintaining the nonlinear gain. The gained results are interpreted in the context of recent experimental findings under propofol-induced anaesthesia.

Cortical balance of excitation and inhibition is regulated by the rate of synaptic activity

Cortical activity is determined by the balance between excitation and inhibition. To examine how shifts in brain activity affect this balance, we recorded spontaneous excitatory and inhibitory synaptic inputs into layer 4 neurons from rat somatosensory cortex while altering the depth of anesthesia. The rate of excitatory and inhibitory events was reduced by ∼50% when anesthesia was deepened. However, whereas both the amplitude and width of inhibitory synaptic events profoundly increased under deep anesthesia, those of excitatory events were unaffected. These effects were found using three different types of anesthetics, suggesting that they are caused by the network state and not by local specific action of the anesthetics. To test our hypothesis that the size of inhibitory events increased because of the decreased rate of synaptic activity under deep anesthesia, we blocked cortical excitation and replayed the slow and fast patterns of inhibitory inputs using intracortical electrical stimulation. Evoked inhibition was larger under low-frequency stimulation, and, importantly, this change occurred regardless of the depth of anesthesia. Hence, shifts in the balance between excitation and inhibition across distinct states of cortical activity can be explained by the rate of inhibitory inputs combined with their short-term plasticity properties, regardless of the actual global brain activity.

Changes in effective connectivity by propofol sedation

Mechanisms of propofol-induced loss of consciousness remain poorly understood. Recent fMRI studies have shown decreases in functional connectivity during unconsciousness induced by this anesthetic agent. Functional connectivity does not provide information of directional changes in the dynamics observed during unconsciousness. The aim of the present study was to investigate, in healthy humans during an auditory task, the changes in effective connectivity resulting from propofol induced loss of consciousness. We used Dynamic Causal Modeling for fMRI (fMRI-DCM) to assess how causal connectivity is influenced by the anesthetic agent in the auditory system. Our results suggest that the dynamic observed in the auditory system during unconsciousness induced by propofol, can result in a mixture of two effects: a local inhibitory connectivity increase and a decrease in the effective connectivity in sensory cortices.

Forebrain HCN1 channels contribute to hypnotic actions of ketamine

BACKGROUND

Ketamine is a commonly used anesthetic, but the mechanistic basis for its clinically relevant actions remains to be determined. The authors previously showed that HCN1 channels are inhibited by ketamine and demonstrated that global HCN1 knockout mice are twofold less sensitive to hypnotic actions of ketamine. Although that work identified HCN1 channels as a viable molecular target for ketamine, it did not determine the relevant neural substrate.

METHODS

To localize the brain region responsible for HCN1-mediated hypnotic actions of ketamine, the authors used a conditional knockout strategy to delete HCN1 channels selectively in excitatory cells of the mouse forebrain. A combination of molecular, immunohistochemical, and cellular electrophysiologic approaches was used to verify conditional HCN1 deletion; a loss-of-righting reflex assay served to ascertain effects of forebrain HCN1 channel ablation on hypnotic actions of ketamine.

RESULTS

In conditional knockout mice, HCN1 channels were selectively deleted in cortex and hippocampus, with expression retained in cerebellum. In cortical pyramidal neurons from forebrain-selective HCN1 knockout mice, effects of ketamine on HCN1-dependent membrane properties were absent; notably, ketamine was unable to evoke membrane hyperpolarization or enhance synaptic inputs. Finally, the EC50 for ketamine-induced loss-of-righting reflex was shifted to significantly higher concentrations (by approximately 31%).

CONCLUSIONS

These data indicate that forebrain principal cells represent a relevant neural substrate for HCN1-mediated hypnotic actions of ketamine. The authors suggest that ketamine inhibition of HCN1 shifts cortical neuron electroresponsive properties to contribute to ketamine-induced hypnosis.

Effects of neural synchrony on surface EEG

It has long been assumed that the surface electroencephalography (EEG) signal depends on both the amplitude and spatial synchronization of underlying neural activity, though isolating their respective contribution remains elusive. To address this, we made simultaneous surface EEG measurements along with intracortical recordings of local field potentials (LFPs) in the primary visual cortex of behaving nonhuman primates. We found that trial-by-trial fluctuations in EEG power could be explained by a linear combination of LFP power and interelectrode temporal synchrony. This effect was observed in both stimulus and stimulus-free conditions and was particularly strong in the gamma range (30-100 Hz). Subsequently, we used pharmacological manipulations to show that neural synchrony can produce a positively modulated EEG signal even when the LFP signal is negatively modulated. Taken together, our results demonstrate that neural synchrony can modulate EEG signals independently of amplitude changes in neural activity. This finding has strong implications for the interpretation of EEG in basic and clinical research, and helps reconcile EEG response discrepancies observed in different modalities (e.g., EEG vs. functional magnetic resonance imaging) and different spatial scales (e.g., EEG vs. intracranial EEG).

Low dose isoflurane exerts opposing effects on neuronal network excitability in neocortex and hippocampus

The anesthetic excitement phase occurring during induction of anesthesia with volatile anesthetics is a well-known phenomenon in clinical practice. However, the physiological mechanisms underlying anesthetic-induced excitation are still unclear. Here we provide evidence from in vitro experiments performed on rat brain slices that the general anesthetic isoflurane at a concentration of about 0.1 mM can enhance neuronal network excitability in the hippocampus, while simultaneously reducing it in the neocortex. In contrast, isoflurane tissue concentrations above 0.3 mM expectedly caused a pronounced reduction in both brain regions. Neuronal network excitability was assessed by combining simultaneous multisite stimulation via a multielectrode array with recording intrinsic optical signals as a measure of neuronal population activity.

The impact of nitrous oxide on electroencephalographic bicoherence during isoflurane anesthesia

BACKGROUND

We previously reported that electroencephalographic (EEG) bicoherence, the degree of phase coupling among the frequency components of a signal, showed 2 peaks during isoflurane anesthesia. Hayashi et al. (Br J Anaesth 2007;99:389-95) also revealed that the peak frequency of bicoherence around 10 Hz increased when ketamine was added. Because nitrous oxide (N(2)O) and ketamine share several common features, they are often treated as the same category of anesthetic. Here, we investigated the effect of N(2)O on EEG bicoherence and other EEG derivatives during isoflurane anesthesia.

METHODS

Twenty patients (aged 34-72 years, ASA physical status I and II) of either gender who underwent elective laparoscopic surgery were included. Raw EEG data, along with EEG-derived parameters, were recorded using an A-1050 Bispectral Index (BIS) monitor and our self-authored Bispectral Analyzer for BIS software. We compared 2 peaks of EEG bicoherence (pBIC-low, around 4 Hz; and pBIC-high, around 10 Hz), as well as BIS and spectral edge frequency 95% (SEF95). Anesthesia was induced with 3 mg · kg(-1) thiopental and 3 μg · kg(-1) fentanyl. After tracheal intubation, anesthesia was maintained with isoflurane (expired concentration at 1.0%), oxygen, and nitrogen. Fentanyl was added and maintained at an estimated effect-site concentration of >1.5 ng · mL(-1). We obtained baseline data 1 hour after induction of anesthesia, then 70% N(2)O was added for 30 minutes.

RESULTS

Before N(2)O, pBIC-low and pBIC-high were 49.3% ± 8.3% and 42.4% ± 11.0%. Ten minutes after starting N(2)O, pBIC-high decreased to 14.9% ± 5.9% (P < 0.001), and it was statistically significantly lower throughout the N(2)O period. Meanwhile, pBIC-low transiently decreased to 37.2% ± 12.8% (P = 0.01) during the early phase of N(2)O administration. Before N(2)O, BIS and SEF95 were 43.2 ± 4.9 and 13.1 ± 2.0 Hz, respectively. Both BIS and SEF95 slightly but statistically significantly decreased during N(2)O administration. Fifteen minutes after starting N(2)O, BIS and SEF95 were 35.7 ± 6.2 (P < 0.001) and 8.6 ± 1.8 Hz (P < 0.001) and they decreased more when large δ waves emerged. Fifteen minutes after stopping N(2)O, BIS, SEF95, as well as pBIC-low and pBIC-high returned to pre-N(2)O values.

CONCLUSION

Dissimilar to the effect of ketamine, N(2)O significantly decreases pBIC-high during isoflurane anesthesia.

Risk of seizures during intraoperative electrocortical stimulation of brain motor areas: a retrospective study on 50 patients

Tumours close to cerebral cortices involved in motor and language functions represent a major challenge for neurosurgeons. Intraoperative neurophysiologic monitoring is useful to gain insight into the anatomy of and the relationship between pathological and normal tissues. In this study we report on the experience of electrocortical stimulation in the surgery of tumours adjacent to the motor cortex in 50 patients under general anaesthesia (26 under propofol, 24 under sevoflurane), and on EMG responses from contralateral muscles. In 18 patients stimulation evoked seizures, which were controlled only with antiepileptic drugs (36%). No difference was found in the incidence of intra-operative seizures between the patients with (10 out of 27) or without (8 out of 23) pre-operative epilepsy (p = 0.8685). The majority of the patients (13 out of 18) with intraoperative seizures were under sevoflurane (p = 0.01) and there was a statistically significant difference in the mean electrical intensity used between the two groups, sevoflurane and propofol, respectively 5.3 ± 1.3 mA and 3.6 ± 2 mA (p = 0.03). Regarding pre-operative anti-epileptic drugs, the use of levitiracetam was associated with a high incidence of intraoperative seizure (5 out of 6 patients). 4 patients developed new, unwanted, permanent neurological deficits, of which 2 had intraoperative seizures controlled only with antiepileptic drugs. Electrocortical stimulation is a powerful tool to understand the functional organization of patients' eloquent areas. Intraoperative epileptic seizures may represent an unwanted complication preventing further stimulation and possibly worsening neurological results. The choice of anaesthetics according to the patients' characteristics, pre-op symptoms and medical therapy is pivotal.

The population firing rate in the presence of GABAergic tonic inhibition in single neurons and application to general anaesthesia

Tonic inhibition has been found experimentally in single neurons and affects the activity of neural populations. This kind of inhibition is supposed to set the background or resting level of neural activity and plays a role in the brains arousal system, e.g. during general anaesthesia. The work shows how to involve tonic inhibition in population rate-coding models by deriving a novel transfer function. The analytical and numerical study of the novel transfer function reveals the impact of tonic inhibition on the population firing rate. Finally, a first application to a recent neural field model for general anaesthesia discusses the origin of the loss of consciousness during anaesthesia.

Identification and characterization of anesthetic targets by mouse molecular genetics approaches

PURPOSE

It is now generally accepted that proteins are the primary targets of general anesthetics. However, the demonstration that the activity of a protein is altered by general anesthetics at clinically relevant concentrations in vitro does not provide direct evidence that this target mediates pharmacological actions of general anesthetics. Here we report on advances that have been made in identifying the contribution of individual ligand-gated ion channels to defined anesthetic endpoints using molecular mouse genetics.

PRINCIPAL FINDINGS

Gamma-aminobutyric acid (GABA)(A) receptor subtypes defined by the presence of the α1, α4, α5, β2, and β3 subunits and two-pore domain potassium channels (TASK-1, TASK-3, and TREK) have been discovered to mediate, at least in part, the hypnotic, immobilizing or amnestic actions of intravenous and volatile general anesthetics. Moreover, using tissues from genetically modified mice, specific functions of GABA(A) receptor subtypes in cortical and spinal neuronal networks were identified.

CONCLUSION

Genetically modified mice have been very useful for research on mechanisms of anesthesia and have contributed to the functional identification of general anesthetic targets and of the role of these targets in neuronal networks.

Effects of the anesthetic agent propofol on neural populations

The neuronal mechanisms of general anesthesia are still poorly understood. Besides several characteristic features of anesthesia observed in experiments, a prominent effect is the bi-phasic change of power in the observed electroencephalogram (EEG), i.e. the initial increase and subsequent decrease of the EEG-power in several frequency bands while increasing the concentration of the anaesthetic agent. The present work aims to derive analytical conditions for this bi-phasic spectral behavior by the study of a neural population model. This model describes mathematically the effective membrane potential and involves excitatory and inhibitory synapses, excitatory and inhibitory cells, nonlocal spatial interactions and a finite axonal conduction speed. The work derives conditions for synaptic time constants based on experimental results and gives conditions on the resting state stability. Further the power spectrum of Local Field Potentials and EEG generated by the neural activity is derived analytically and allow for the detailed study of bi-spectral power changes. We find bi-phasic power changes both in monostable and bistable system regime, affirming the omnipresence of bi-spectral power changes in anesthesia. Further the work gives conditions for the strong increase of power in the δ-frequency band for large propofol concentrations as observed in experiments.

Gain of the human dura in vivo and its effects on invasive brain signal feature detection

Invasive brain signal recordings generally rely on bioelectrodes implanted on the cortex underneath the dura. Subdural recordings have strong advantages in terms of bandwidth, spatial resolution and signal quality. However, subdural electrodes also have the drawback of compromising the long-term stability of such implants and heighten the risk of infection. Epidurally implanted electrodes might provide a viable alternative to subdural electrodes, offering a compromise between signal quality and invasiveness. Determining the feasibility of epidural electrode implantation for e.g., clinical research, brain-computer interfacing (BCI) and cognitive experiments, requires the characterization of the electrical properties of the dura, and its effect on signal feature detection. In this paper we report measurements of brain signal attenuation by the human dura in vivo. In addition, we use signal detection theory to study how the presence of the dura between the sources and the recording electrodes affects signal power features in motor BCI experiments. For noise levels typical of clinical brain signal recording equipment, we observed no detrimental effects on signal feature detection due to the dura. Subdural recordings were found to be more robust with respect to increased instrumentation noise level as compared to their epidural counterpart nonetheless. Our findings suggest that epidural electrode implantation is a viable alternative to subdural implants from the feature detection viewpoint.

HCN1 channel subunits are a molecular substrate for hypnotic actions of ketamine

Ketamine has important anesthetic, analgesic, and psychotropic actions. It is widely believed that NMDA receptor inhibition accounts for ketamine actions, but there remains a dearth of behavioral evidence to support this hypothesis. Here, we present an alternative, behaviorally relevant molecular substrate for anesthetic effects of ketamine: the HCN1 pacemaker channels that underlie a neuronal hyperpolarization-activated cationic current (I(h)). Ketamine caused subunit-specific inhibition of recombinant HCN1-containing channels and neuronal I(h) at clinically relevant concentrations; the channels were more potently inhibited by S-(+)-ketamine than racemic ketamine, consistent with anesthetic actions of the compounds. In cortical pyramidal neurons from wild-type, but not HCN1 knock-out mice, ketamine induced membrane hyperpolarization and enhanced dendritosomatic synaptic coupling; both effects are known to promote cortical synchronization and support slow cortical rhythms, like those accompanying anesthetic-induced hypnosis. Accordingly, we found that the potency for ketamine to provoke a loss-of-righting reflex, a behavioral correlate of hypnosis, was strongly reduced in HCN1 knock-out mice. In addition, hypnotic sensitivity to two other intravenous anesthetics in HCN1 knock-out mice matched effects on HCN1 channels; propofol selectively inhibited HCN1 channels and propofol sensitivity was diminished in HCN1 knock-out mice, whereas etomidate had no effect on HCN1 channels and hypnotic sensitivity to etomidate was unaffected by HCN1 gene deletion. These data advance HCN1 channels as a novel molecular target for ketamine, provide a plausible neuronal mechanism for enhanced cortical synchronization during anesthetic-induced hypnosis and suggest that HCN1 channels might contribute to other unexplained actions of ketamine.

Subunit-specific effects of isoflurane on neuronal Ih in HCN1 knockout mice

The ionic mechanisms that contribute to general anesthetic actions have not been elucidated, although increasing evidence has pointed to roles for subthreshold ion channels, such as the HCN channels underlying the neuronal hyperpolarization-activated cationic current (Ih). Here, we used conventional HCN1 knockout mice to test directly the contributions of specific HCN subunits to effects of isoflurane, an inhalational anesthetic, on membrane and integrative properties of motor and cortical pyramidal neurons in vitro. Compared with wild-type mice, residual Ih from knockout animals was smaller in amplitude and presented with HCN2-like properties. Inhibition of Ih by isoflurane previously attributed to HCN1 subunit-containing channels (i.e., a hyperpolarizing shift in half-activation voltage [V1/2]) was absent in neurons from HCN1 knockout animals; the remaining inhibition of current amplitude could be attributed to effects on residual HCN2 channels. We also found that isoflurane increased temporal summation of excitatory postsynaptic potentials (EPSPs) in cortical neurons from wild-type mice; this effect was predicted by simulation of anesthetic-induced dendritic Ih inhibition, which also revealed more prominent summation accompanying shifts in V1/2 (an HCN1-like effect) than decreased current amplitude (an HCN2-like effect). Accordingly, anesthetic-induced EPSP summation was not observed in cortical cells from HCN1 knockout mice. In wild-type mice, the enhanced synaptic summation observed with low concentrations of isoflurane contributed to a net increase in cortical neuron excitability. In summary, HCN channel subunits account for distinct anesthetic effects on neuronal membrane properties and synaptic integration; inhibition of HCN1 in cortical neurons may contribute to the synaptically mediated slow-wave cortical synchronization that accompanies anesthetic-induced hypnosis.

Discrimination of auditory stimuli during isoflurane anesthesia

Deep isoflurane anesthesia initiates a burst suppression pattern in which high-amplitude bursts are preceded by periods of nearly silent electroencephalogram. The burst suppression ratio (BSR) is the percentage of suppression (silent electroencephalogram) during the burst suppression pattern and is one parameter used to assess anesthesia depth. We investigated cortical burst activity in rats in response to different auditory stimuli presented during the burst suppression state. We noted a rapid appearance of bursts and a significant decrease in the BSR during stimulation. The BSR changes were distinctive for the different stimuli applied, and the BSR decreased significantly more when stimulated with a voice familiar to the rat as compared with an unfamiliar voice. These results show that the cortex can show differential sensory responses during deep isoflurane anesthesia.

Effect of common anesthetics on dendritic properties in layer 5 neocortical pyramidal neurons

Understanding the impact of active dendritic properties on network activity in vivo has so far been restricted to studies in anesthetized animals. However, to date no study has been made to determine the direct effect of the anesthetics themselves on dendritic properties. Here, we investigated the effects of three types of anesthetics commonly used for animal experiments (urethane, pentobarbital and ketamine/xylazine). We investigated the generation of calcium spikes, the propagation of action potentials (APs) along the apical dendrite and the somatic firing properties in the presence of anesthetics in vitro using dual somatodendritic whole cell recordings. Calcium spikes were evoked with dendritic current injection and high-frequency trains of APs at the soma. Surprisingly, we found that the direct actions of anesthetics on calcium spikes were very different. Two anesthetics (urethane and pentobarbital) suppressed dendritic calcium spikes in vitro, whereas a mixture of ketamine and xylazine enhanced them. Propagation of spikes along the dendrite was not significantly affected by any of the anesthetics but there were various changes in somatic firing properties that were highly dependent on the anesthetic. Last, we examined the effects of anesthetics on calcium spike initiation and duration in vivo using high-frequency trains of APs generated at the cell body. We found the same anesthetic-dependent direct effects in addition to an overall reduction in dendritic excitability in anesthetized rats with all three anesthetics compared with the slice preparation.

At clinically relevant concentrations the anaesthetic/amnesic thiopental but not the anticonvulsant phenobarbital interferes with hippocampal sharp wave-ripple complexes

Background

Many sedative agents, including anesthetics, produce explicit memory impairment by largely unknown mechanisms. Sharp-wave ripple (SPW-R) complexes are network activity thought to represent the neuronal substrate for information transfer from the hippocampal to neocortical circuits, contributing to the explicit memory consolidation. In this study we examined and compared the actions of two barbiturates with distinct amnesic actions, the general anesthetic thiopental and the anticonvulsant phenobarbital, on in vitro SPW-R activity.

Results

Using an in vitro model of SPW-R activity we found that thiopental (50–200 μM) significantly and concentration-dependently reduced the incidence of SPW-R events (it increased the inter-event period by 70–430 %). At the concentration of 25 μM, which clinically produces mild sedation and explicit memory impairment, thiopental significantly reduced the quantity of ripple oscillation (it reduced the number of ripples and the duration of ripple episodes by 20 ± 5%, n = 12, P < 0.01), and suppressed the rhythmicity of SPWs by 43 ± 15% (n = 6, P < 0.05). The drug disrupted the synchrony of SPWs within the CA1 region at 50 μM (by 19 ± 12%; n = 5, P < 0.05). Similar effects of thiopental were observed at higher concentrations. Thiopental did not affect the frequency of ripple oscillation at any of the concentrations tested (10–200 μM). Furthermore, the drug significantly prolonged single SPWs at concentrations ≥50 μM (it increased the half-width and the duration of SPWs by 35–90 %). Thiopental did not affect evoked excitatory synaptic potentials and its results on SPW-R complexes were also observed under blockade of NMDA receptors. Phenobarbital significantly accelerated SPWs at 50 and 100 μM whereas it reduced their rate at 200 and 400 μM. Furthermore, it significantly prolonged SPWs, reduced their synchrony and reduced the quantity of ripples only at the clinically very high concentration of 400 μM, reported to affect memory.

Conclusion

We hypothesize that thiopental, by interfering with SPW-R activity, through enhancement of the GABA_A receptor-mediated transmission, affects memory processes which involve hippocampal circuit activation. The quantity but not the frequency of ripple oscillation was affected by the drug.

Ketamine, an NMDA-antagonist, increases the oscillatory frequencies of alpha-peaks on the electroencephalographic power spectrum

BACKGROUND

Ketamine, an N-methyl-D-aspartate (NMDA) antagonist, is known to activate the electroencephalogram (EEG), despite its sedative effects. Spindle oscillations are known to be related to the sedative actions of the reticular thalamic nucleus with links to thalamocortical neurons. This study was designed to examine the effect of ketamine on the spindle oscillations to understand the simultaneous sedative effect and EEG activation that occurs with ketamine, by comparing the EEG in emergence.

METHODS

Anesthesia was induced with propofol using a target-controlled infusion (TCI) system (3.5 microg/ml). Seventeen patients, scheduled for non-cranial surgery under general anesthesia combined with epidural anesthesia, were randomly divided into two groups: (i) anesthesia was maintained with TCI-propofol alone (n= 8) and (ii) anesthesia was maintained with TCI-propofol and intravenously administered ketamine (n= 9). The EEG was continuously monitored and EEG indices and power spectra were determined.

RESULTS

Propofol alone caused the alpha-peaks of the power spectra to occur at an average frequency of 10.4 +/- 0.9 Hz; the addition of ketamine shifted the peaks to higher frequencies of 15.1 +/- 1.4 Hz (P < 0.05). On the other hand, when the EEG was activated by discontinuation of propofol, the corresponding alpha-peaks disappeared.

CONCLUSIONS

Ketamine increased the frequencies of alpha-spindle waves induced by propofol, but did not block their formations. The phenomena have the possibility to underlie the cooperative effect between propofol and ketamine concerning sedation and anesthesia.

Bispectral index is a topographically dependent variable in patients receiving propofol anaesthesia

BACKGROUND

As very strong agreement has been reported between bispectral index (BIS) values measured from the occipital and frontal skull areas, we compared BIS values measured from central and parietal areas with those from frontal area to investigate whether BIS is really a topographically dependent or topographically independent variable.

METHODS

Twenty patients, ASA I-II, non-obese, aged 18-62 yr and with no neurological disorders were enrolled. Based on the 10-20 international landmarks, five silver dome electrodes were positioned: F7, C3, P7, Cz (common reference) and Fp1 (ground). Using frontal (F7-Cz), central (C3-Cz) and parietal (P7-Cz) electrode montages, the corresponding BIS values were simultaneously recorded with an Aspect A-1000 monitor (software v3.12). The BIS values were recorded at the propofol concentration allowing laryngeal mask insertion, which was maintained during the 10 min data collection period in absence of additional external stimuli. Data were analysed using the Kruskall-Wallis, Wilcoxon paired sign with Bonferroni correction, Bland-Altman and linear correlation tests.

RESULTS

At the predicted effect target propofol concentration 4-8 microg ml(-1), the 10 min mean BIS (median [min-max]) were 32 [20-44], 46 [28-68] and 58 [41-72] for the frontal, central and parietal leads, respectively. Differences between these BIS recordings were statistically significant (P<0.0001, Kruskall-Wallis; P<0.005, Wilcoxon paired sign test).

CONCLUSIONS

The present results provide evidence that BIS index is a topographically dependent variable in patients receiving propofol anaesthesia.

Measuring anesthesia in children using the EEG

Advances in electroencephalogram (EEG) processing have produced new interest in measuring anesthesia using the EEG. There are a number of EEG-based anesthesia 'depth' monitors now available and their use in pediatric anesthesia is increasing. Although these monitors have been extensively studied in adults, there are relatively few studies examining their validity or use in children. To some extent we must rely on adult data. However, extrapolation of data from adults to children requires an in depth understanding of the physiology behind the data. The first question is what is being measured. What is anesthesia? A model of anesthesia has several components with arousal as a core component. Arousal can be linked to clinical observations, and correlates with anatomical and physiological studies. The EEG has characteristics that fairly consistently change with arousal during anesthesia, but the relationship between arousal and the EEG is imprecise and drug dependent. This relationship is the basis for using the EEG to measure anesthesia and provides only an indirect measure of consciousness and memory formation. A good understanding of how the EEG is related to anesthesia is essential when interpreting the EEG during anesthesia, and especially when extending the use of the EEG to measure anesthesia in children. Physiological studies in adults and children indicate that EEG-derived anesthesia depth monitors can provide an imprecise and drug-dependent measure of arousal. Although the outputs from these monitors do not closely represent any true physiological entity, they can be used as guides for anesthesia and in so doing have improved outcomes in adults. In older children the physiology, anatomy and clinical observations indicate the performance of the monitors may be similar to that in adults, although the clinical relevance of outcomes may be different. In infants their use cannot yet be supported in theory or in practice.

HCN subunit-specific and cAMP-modulated effects of anesthetics on neuronal pacemaker currents

General anesthetics have been a mainstay of surgical practice for more than 150 years, but the mechanisms by which they mediate their important clinical actions remain unclear. Ion channels represent important anesthetic targets, and, although GABA(A) receptors have emerged as major contributors to sedative, immobilizing, and hypnotic effects of intravenous anesthetics, a role for those receptors is less certain in the case of inhalational anesthetics. The neuronal hyperpolarization-activated pacemaker current (Ih) is essential for oscillatory and integrative properties in numerous cell types. Here, we show that clinically relevant concentrations of inhalational anesthetics modulate neuronal Ih and the corresponding HCN channels in a subunit-specific and cAMP-dependent manner. Anesthetic inhibition of Ih involves a hyperpolarizing shift in voltage dependence of activation and a decrease in maximal current amplitude; these effects can be ascribed to HCN1 and HCN2 subunits, respectively, and both actions are recapitulated in heteromeric HCN1-HCN2 channels. Mutagenesis and simulations suggest that apparently distinct actions of anesthetics on V(1/2) and amplitude represent different manifestations of a single underlying mechanism (i.e., stabilization of channel closed state), with the predominant action determined by basal inhibition imposed by individual subunit C-terminal domains and relieved by cAMP. These data reveal a molecular basis for multiple actions of anesthetics on neuronal HCN channels, highlight the importance of proximal C terminus in modulation of HCN channel gating by diverse agents, and advance neuronal pacemaker channels as potentially relevant targets for clinical actions of inhaled anesthetics.

Interactions of anesthetics with their targets: non-specific, specific or both?

What makes a general anesthetic a general anesthetic? We shall review first what general anesthesia is all about and which drugs are being used as anesthetics. There is neither a unique definition of general anesthesia nor any consensus on how to measure it. Diverse drugs and combinations of drugs generate general anesthetic states of sometimes very different clinical quality. Yet the principal drugs are still considered to belong to the same class of 'general anesthetics'. Effective concentrations of inhalation anesthetics are in the high micromolar range and above, and even for intravenous anesthetics they do not go below the micromolar range. At these concentrations, many molecular and higher level targets are affected by inhalation anesthetics, fewer probably by intravenous anesthetics. The only physicochemical characteristic shared by anesthetics is the correlation of their anesthetic potencies with hydrophobicity. These correlations depend on the group of general anesthetics considered. In this review, anesthetic potencies for many different targets are plotted against octanol/water partition coefficients as measure of hydrophobicity. Qualitatively, similar correlations result, suggesting several but weak interactions with proteins as being characteristic of anesthetic actions. The polar interactions involved are weak, being roughly equal in magnitude to hydrophobic interactions. Generally, intravenous anesthetics are noticeably more potent than inhalation anesthetics. They differ considerably more between each other in their interactions with various targets than inhalation anesthetics do, making it difficult to come to a decision which of these should be used in future studies as representative 'prototypical general anesthetics'.

Modeling the effects of anesthesia on the electroencephalogram

Changes to the electroencephalogram (EEG) observed during general anesthesia are modeled with a physiological mean field theory of electrocortical activity. To this end a parametrization of the postsynaptic impulse response is introduced which takes into account pharmacological effects of anesthetic agents on neuronal ligand-gated ionic channels. Parameter sets for this improved theory are then identified which respect known anatomical constraints and predict mean firing rates and power spectra typically encountered in human subjects. Through parallelized simulations of the eight nonlinear, two-dimensional partial differential equations on a grid representing an entire human cortex, it is demonstrated that linear approximations are sufficient for the prediction of a range of quantitative EEG variables. More than 70,000 plausible parameter sets are finally selected and subjected to a simulated induction with the stereotypical inhaled general anesthetic isoflurane. Thereby 86 parameter sets are identified that exhibit a strong "biphasic" rise in total power, a feature often observed in experiments. A sensitivity study suggests that this "biphasic" behavior is distinguishable even at low agent concentrations. Finally, our results are briefly compared with previous work by other groups and an outlook on future fits to experimental data is provided.

Propofol suppresses synaptic responsiveness of somatosensory relay neurons to excitatory input by potentiating GABA(A) receptor chloride channels

Propofol is a widely used intravenous general anesthetic. Propofol-induced unconsciousness in humans is associated with inhibition of thalamic activity evoked by somatosensory stimuli. However, the cellular mechanisms underlying the effects of propofol in thalamic circuits are largely unknown. We investigated the influence of propofol on synaptic responsiveness of thalamocortical relay neurons in the ventrobasal complex (VB) to excitatory input in mouse brain slices, using both current- and voltage-clamp recording techniques. Excitatory responses including EPSP temporal summation and action potential firing were evoked in VB neurons by electrical stimulation of corticothalamic fibers or pharmacological activation of glutamate receptors. Propofol (0.6 - 3 microM) suppressed temporal summation and spike firing in a concentration-dependent manner. The thalamocortical suppression was accompanied by a marked decrease in both EPSP amplitude and input resistance, indicating that a shunting mechanism was involved. The propofol-mediated thalamocortical suppression could be blocked by a GABAA receptor antagonist or chloride channel blocker, suggesting that postsynaptic GABAA receptors in VB neurons were involved in the shunting inhibition. GABAA receptor-mediated inhibitory postsynaptic currents (IPSCs) were evoked in VB neurons by electrical stimulation of the reticular thalamic nucleus. Propofol markedly increased amplitude, decay time, and charge transfer of GABAA IPSCs. The results demonstrated that shunting inhibition of thalamic somatosensory relay neurons by propofol at clinically relevant concentrations is primarily mediated through the potentiation of the GABAA receptor chloride channel-mediated conductance, and such inhibition may contribute to the impaired thalamic responses to sensory stimuli seen during propofol-induced anesthesia.

Isoflurane modulates glutamatergic and GABAergic neurotransmission in the amygdala

Attempts have been made to attribute the particular features of general anaesthesia such as hypnosis, analgesia, amnesia and autonomic stability to certain brain regions. In the present study, we examined the effects of the commonplace volatile anaesthetic isoflurane on synaptic transmission in an in vitro slice preparation of the murine amygdala. Despite the established role of this limbic structure in the formation of aversive memories, conditioned fear and anxiety, as well as pain processing and regulation of sympathetic tone, the influence of volatile anaesthetics on synaptic signalling has not yet been investigated in this region of the brain. Evoked postsynaptic currents were monitored from principal neurons in the basolateral nucleus of the amygdala by means of patch-clamp recording. The mixed postsynaptic currents were mediated by non-NMDA, NMDA, GABA A and GABA B receptors. Isoflurane added to the perfusion medium reduced the strength of synaptic signalling following the activation of non-NMDA, NMDA, and GABA B receptors, whereas the GABA A receptor-mediated responses were enhanced. The overall reduction of neuronal excitability was also reflected in a reduction of field potential amplitudes. Isoflurane neither changed the membrane resting potential nor the input resistance of principal neurons in the amygdala. The present results may contribute to the understanding of how stress reactions and long-lasting neuroplastic processes are suppressed under general anaesthesia.