Decreased Thalamic Activity Is a Correlate for Disconnectedness during Anesthesia with Propofol, Dexmedetomidine and Sevoflurane But Not S-Ketamine.

Establishing the neural mechanisms responsible for the altered global states of consciousness during anesthesia and dissociating these from other drug-related effects remains a challenge in consciousness research. We investigated differences in brain activity between connectedness and disconnectedness by administering various anesthetics at concentrations designed to render 50% of the subjects unresponsive. One hundred and sixty healthy male subjects were randomized to receive either propofol (1.7 µg/ml; n = 40), dexmedetomidine (1.5 ng/ml; n = 40), sevoflurane (0.9% end-tidal; n = 40), S-ketamine (0.75 µg/ml; n = 20), or saline placebo (n = 20) for 60 min using target-controlled infusions or vaporizer with end-tidal monitoring. Disconnectedness was defined as unresponsiveness to verbal commands probed at 2.5-min intervals and unawareness of external events in a postanesthesia interview. High-resolution positron emission tomography (PET) was used to quantify regional cerebral metabolic rates of glucose (CMRglu) utilization. Contrasting scans where the subjects were classified as connected and responsive versus disconnected and unresponsive revealed that for all anesthetics, except S-ketamine, the level of thalamic activity differed between these states. A conjunction analysis across the propofol, dexmedetomidine and sevoflurane groups confirmed the thalamus as the primary structure where reduced metabolic activity was related to disconnectedness. Widespread cortical metabolic suppression was observed when these subjects, classified as either connected or disconnected, were compared with the placebo group, suggesting that these findings may represent necessary but alone insufficient mechanisms for the change in the state of consciousness.SIGNIFICANCE STATEMENT Experimental anesthesia is commonly used in the search for measures of brain function which could distinguish between global states of consciousness. However, most previous studies have not been designed to separate effects related to consciousness from other effects related to drug exposure. We employed a novel study design to disentangle these effects by exposing subjects to predefined EC50 doses of four commonly used anesthetics or saline placebo. We demonstrate that state-related effects are remarkably limited compared with the widespread cortical effects related to drug exposure. In particular, decreased thalamic activity was associated with disconnectedness with all used anesthetics except for S-ketamine.

Foundations of Human Consciousness: Imaging the Twilight Zone.

What happens in the brain when conscious awareness of the surrounding world fades? We manipulated consciousness in two experiments in a group of healthy males and measured brain activity with positron emission tomography. Measurements were made during wakefulness, escalating and constant levels of two anesthetic agents (experiment 1, n = 39), and during sleep-deprived wakefulness and non-rapid eye movement sleep (experiment 2, n = 37). In experiment 1, the subjects were randomized to receive either propofol or dexmedetomidine until unresponsiveness. In both experiments, forced awakenings were applied to achieve rapid recovery from an unresponsive to a responsive state, followed by immediate and detailed interviews of subjective experiences during the preceding unresponsive condition. Unresponsiveness rarely denoted unconsciousness, as the majority of the subjects had internally generated experiences. Unresponsive anesthetic states and verified sleep stages, where a subsequent report of mental content included no signs of awareness of the surrounding world, indicated a disconnected state. Functional brain imaging comparing responsive and connected versus unresponsive and disconnected states of consciousness during constant anesthetic exposure revealed that activity of the thalamus, cingulate cortices, and angular gyri are fundamental for human consciousness. These brain structures were affected independent from the pharmacologic agent, drug concentration, and direction of change in the state of consciousness. Analogous findings were obtained when consciousness was regulated by physiological sleep. State-specific findings were distinct and separable from the overall effects of the interventions, which included widespread depression of brain activity across cortical areas. These findings identify a central core brain network critical for human consciousness.SIGNIFICANCE STATEMENT Trying to understand the biological basis of human consciousness is currently one of the greatest challenges of neuroscience. While the loss and return of consciousness regulated by anesthetic drugs and physiological sleep are used as model systems in experimental studies on consciousness, previous research results have been confounded by drug effects, by confusing behavioral "unresponsiveness" and internally generated consciousness, and by comparing brain activity levels across states that differ in several other respects than only consciousness. Here, we present carefully designed studies that overcome many previous confounders and for the first time reveal the neural mechanisms underlying human consciousness and its disconnection from behavioral responsiveness, both during anesthesia and during normal sleep, and in the same study subjects.

Impact of Global Mean Normalization on Regional Glucose Metabolism in the Human Brain.

Because the human brain consumes a disproportionate fraction of the resting body's energy, positron emission tomography (PET) measurements of absolute glucose metabolism (CMRglc) can serve as disease biomarkers. Global mean normalization (GMN) of PET data reveals disease-based differences from healthy individuals as fractional changes across regions relative to a global mean. To assess the impact of GMN applied to metabolic data, we compared CMRglc with and without GMN in healthy awake volunteers with eyes closed (i.e., control) against specific physiological/clinical states, including healthy/awake with eyes open, healthy/awake but congenitally blind, healthy/sedated with anesthetics, and patients with disorders of consciousness. Without GMN, global CMRglc alterations compared to control were detected in all conditions except in congenitally blind where regional CMRglc variations were detected in the visual cortex. However, GMN introduced regional and bidirectional CMRglc changes at smaller fractions of the quantitative delocalized changes. While global information was lost with GMN, the quantitative approach (i.e., a validated method for quantitative baseline metabolic activity without GMN) not only preserved global CMRglc alterations induced by opening eyes, sedation, and varying consciousness but also detected regional CMRglc variations in the congenitally blind. These results caution the use of GMN upon PET-measured CMRglc data in health and disease.

Using Positron Emission Tomography in Revealing the Mystery of General Anesthesia: Study Design Challenges and Opportunities.

Functional neuroimaging with positron emission tomography (PET) is one of the cornerstones for studying the central nervous system effects of general anesthetics and anesthesia mechanisms. General anesthesia offers a unique and safe way to directly manipulate consciousness, and can thus be used as a powerful research tool to study the neurobiology of human consciousness. In this chapter, we will address the possibilities of PET imaging in revealing the mysteries of general anesthesia and anesthetic induced unconsciousness and summarize some of the recent advancements in the field. Importantly, we will discuss possible ways to separate brain activity changes associated with the changing level of consciousness from the concentration or dose-dependent direct or indirect drug effects on the brain. We will try to demonstrate how state-of-the-art clinical pharmacology, use of specific anesthetic drugs, and innovative study design solutions could be utilized.

Shaker-related potassium channels in the central medial nucleus of the thalamus are important molecular targets for arousal suppression by volatile general anesthetics.

The molecular targets and neural circuits that underlie general anesthesia are not fully elucidated. Here, we directly demonstrate that Kv1-family (Shaker-related) delayed rectifier K(+) channels in the central medial thalamic nucleus (CMT) are important targets for volatile anesthetics. The modulation of Kv1 channels by volatiles is network specific as microinfusion of ShK, a potent inhibitor of Kv1.1, Kv1.3, and Kv1.6 channels, into the CMT awakened sevoflurane-anesthetized rodents. In heterologous expression systems, sevoflurane, isoflurane, and desflurane at subsurgical concentrations potentiated delayed rectifier Kv1 channels at low depolarizing potentials. In mouse thalamic brain slices, sevoflurane inhibited firing frequency and delayed the onset of action potentials in CMT neurons, and ShK-186, a Kv1.3-selective inhibitor, prevented these effects. Our findings demonstrate the exquisite sensitivity of delayed rectifier Kv1 channels to modulation by volatile anesthetics and highlight an arousal suppressing role of Kv1 channels in CMT neurons during the process of anesthesia.

Evolution of consciousness: phylogeny, ontogeny, and emergence from general anesthesia.

Are animals conscious? If so, when did consciousness evolve? We address these long-standing and essential questions using a modern neuroscientific approach that draws on diverse fields such as consciousness studies, evolutionary neurobiology, animal psychology, and anesthesiology. We propose that the stepwise emergence from general anesthesia can serve as a reproducible model to study the evolution of consciousness across various species and use current data from anesthesiology to shed light on the phylogeny of consciousness. Ultimately, we conclude that the neurobiological structure of the vertebrate central nervous system is evolutionarily ancient and highly conserved across species and that the basic neurophysiologic mechanisms supporting consciousness in humans are found at the earliest points of vertebrate brain evolution. Thus, in agreement with Darwin's insight and the recent "Cambridge Declaration on Consciousness in Non-Human Animals," a review of modern scientific data suggests that the differences between species in terms of the ability to experience the world is one of degree and not kind.

Event-related functional magnetic resonance imaging of a low dose of dexmedetomidine that impairs long-term memory.

BACKGROUND: Work suggests the amnesia from dexmedetomidine (an α2-adrenergic agonist) is caused by a failure of information to be encoded into long-term memory and that dexmedetomidine might differentially affect memory for emotionally arousing material. We investigated these issues in humans using event-related neuroimaging to reveal alterations in brain activity and subsequent memory effects associated with drug exposure. METHODS: Forty-eight healthy volunteers received a computer-controlled infusion of either placebo or low-dose dexmedetomidine (target = 0.15 ng/ml plasma) during neuroimaging while they viewed and rated 80 emotionally arousing (e.g., graphic war wound) and 80 nonarousing neutral (e.g., cup) pictures for emotional arousal content. Long-term picture memory was tested 4 days later without neuroimaging. Imaging data were analyzed for drug effects, emotional processing differences, and memory-related changes with statistical parametric mapping-8. RESULTS: Dexmedetomidine impaired overall (mean ± SEM) picture memory (placebo: 0.58 ± 0.03 vs. dexmedetomidine: 0.45 ± 0.03, P = 0.001), but did not differentially modulate memory as a function of item arousal. Arousing pictures were better remembered for both groups. Dexmedetomidine had regionally heterogeneous effects on brain activity, primarily decreasing it in the cortex and increasing it in thalamic and posterior hippocampal regions. Nevertheless, a single subsequent memory effect for item memory common to both groups was identified only in the left hippocampus/amygdala. Much of this effect was found to be larger for the placebo than dexmedetomidine group. CONCLUSION: Dexmedetomidine impaired long-term picture memory, but did not disproportionately block memory for emotionally arousing items. The memory impairment on dexmedetomidine corresponds with a weakened hippocampal subsequent memory effect.

Effect of synthetic aß peptide oligomers and fluorinated solvents on Kv1.3 channel properties and membrane conductance.

The impact of synthetic amyloid ß (1-42) (Aß(1-42)) oligomers on biophysical properties of voltage-gated potassium channels Kv 1.3 and lipid bilayer membranes (BLMs) was quantified for protocols using hexafluoroisopropanol (HFIP) or sodium hydroxide (NaOH) as solvents prior to initiating the oligomer formation. Regardless of the solvent used Aß(1-42) samples contained oligomers that reacted with the conformation-specific antibodies A11 and OC and had similar size distributions as determined by dynamic light scattering. Patch-clamp recordings of the potassium currents showed that synthetic Aß(1-42) oligomers accelerate the activation and inactivation kinetics of Kv 1.3 current with no significant effect on current amplitude. In contrast to oligomeric samples, freshly prepared, presumably monomeric, Aß(1-42) solutions had no effect on Kv 1.3 channel properties. Aß(1-42) oligomers had no effect on the steady-state current (at -80 mV) recorded from Kv 1.3-expressing cells but increased the conductance of artificial BLMs in a dose-dependent fashion. Formation of amyloid channels, however, was not observed due to conditions of the experiments. To exclude the effects of HFIP (used to dissolve lyophilized Aß(1-42) peptide), and trifluoroacetic acid (TFA) (used during Aß(1-42) synthesis), we determined concentrations of these fluorinated compounds in the stock Aß(1-42) solutions by (19)F NMR. After extensive evaporation, the concentration of HFIP in the 100× stock Aß(1-42) solutions was ∼1.7 µM. The concentration of residual TFA in the 70× stock Aß(1-42) solutions was ∼20 µM. Even at the stock concentrations neither HFIP nor TFA alone had any effect on potassium currents or BLMs. The Aß(1-42) oligomers prepared with HFIP as solvent, however, were more potent in the electrophysiological tests, suggesting that fluorinated compounds, such as HFIP or structurally-related inhalational anesthetics, may affect Aß(1-42) aggregation and potentially enhance ability of oligomers to modulate voltage-gated ion channels and biological membrane properties.

Returning from oblivion: imaging the neural core of consciousness.

One of the greatest challenges of modern neuroscience is to discover the neural mechanisms of consciousness and to explain how they produce the conscious state. We sought the underlying neural substrate of human consciousness by manipulating the level of consciousness in volunteers with anesthetic agents and visualizing the resultant changes in brain activity using regional cerebral blood flow imaging with positron emission tomography. Study design and methodology were chosen to dissociate the state-related changes in consciousness from the effects of the anesthetic drugs. We found the emergence of consciousness, as assessed with a motor response to a spoken command, to be associated with the activation of a core network involving subcortical and limbic regions that become functionally coupled with parts of frontal and inferior parietal cortices upon awakening from unconsciousness. The neural core of consciousness thus involves forebrain arousal acting to link motor intentions originating in posterior sensory integration regions with motor action control arising in more anterior brain regions. These findings reveal the clearest picture yet of the minimal neural correlates required for a conscious state to emerge.

Thalamic microinfusion of antibody to a voltage-gated potassium channel restores consciousness during anesthesia.

BACKGROUND: The Drosophila Shaker mutant fruit-fly, with its malfunctioning voltage-gated potassium channel, exhibits anesthetic requirements that are more than twice normal. Shaker mutants with an abnormal Kv1.2 channel also demonstrate significantly reduced sleep. Given the important role the thalamus plays in both sleep and arousal, the authors investigated whether localized central medial thalamic (CMT) microinfusion of an antibody designed to block the pore of the Kv1.2 channel might awaken anesthetized rats. METHODS: Male Sprague-Dawley rats were implanted with a cannula aimed at the CMT or lateral thalamus. One week later, unconsciousness was induced with either desflurane (3.6 +/- 0.2%; n = 55) or sevoflurane (1.2 +/- 0.1%; n = 51). Arousal effects of a single 0.5-microl infusion of Kv1.2 potassium channel blocking antibody (0.1- 0.2 mg/ml) or a control infusion of Arc-protein antibody (0.2 mg/ml) were then determined. RESULTS: The Kv1.2 antibody, but not the control antibody, temporarily restored consciousness in 17% of all animals and in 75% of those animals where infusions occurred within the CMT (P < 0.01 for each anesthetic). Lateral thalamic infusions showed no effects. Consciousness returned on average (+/- SD) 170 +/- 99 s after infusion and lasted a median time of 398 s (interquartile range: 279-510 s). Temporary seizures, without apparent consciousness, predominated in 33% of all animals. CONCLUSIONS: These findings support the idea that the CMT plays a role in modulating levels of arousal during anesthesia and further suggest that voltage-gated potassium channels in the CMT may contribute to regulating arousal or may even be relevant targets of anesthetic action.

Consciousness and anesthesia.

When we are anesthetized, we expect consciousness to vanish. But does it always? Although anesthesia undoubtedly induces unresponsiveness and amnesia, the extent to which it causes unconsciousness is harder to establish. For instance, certain anesthetics act on areas of the brain's cortex near the midline and abolish behavioral responsiveness, but not necessarily consciousness. Unconsciousness is likely to ensue when a complex of brain regions in the posterior parietal area is inactivated. Consciousness vanishes when anesthetics produce functional disconnection in this posterior complex, interrupting cortical communication and causing a loss of integration; or when they lead to bistable, stereotypic responses, causing a loss of information capacity. Thus, anesthetics seem to cause unconsciousness when they block the brain's ability to integrate information.

Neuroimaging analysis of an anesthetic gas that blocks human emotional memory.

It is hypothesized that emotional arousal modulates long-term memory consolidation through the amygdala. Gaseous anesthetic agents are among the most potent drugs that cause temporary amnesia, yet the effects of inhalational anesthesia on human emotional memory processing remain unknown. To study this, two experiments were performed with the commonly used inhalational anesthetic sevoflurane. In experiment 1, volunteers responded to a series of emotional and neutral slides while under various subanesthetic doses of sevoflurane or placebo (no anesthesia). One week later, a mnemonic boost for emotionally arousing stimuli was evident in the placebo, 0.1%, and 0.2% sevoflurane groups, as measured with a recognition test. However, the mnemonic boost was absent in subjects who received 0.25% sevoflurane. Subsequently, in experiment 2, glucose PET assessed brain-state-related activity of subjects exposed to 0.25% sevoflurane. Structural equation modeling of the PET data revealed that 0.25% sevoflurane suppressed amygdala to hippocampal effective connectivity. The behavioral results show that 0.25% sevoflurane blocks emotional memory, and connectivity results demonstrate that this dose of sevoflurane suppresses the effective influence of the amygdala. Collectively, the findings support the hypothesis that the amygdala mediates memory modulation by demonstrating that suppressed amygdala effectiveness equates with a loss of emotional memory.

Thalamic microinjection of nicotine reverses sevoflurane-induced loss of righting reflex in the rat.

BACKGROUND: Neuronal nicotinic acetylcholine receptors are both potently inhibited by anesthetics and densely expressed in the thalamus. Brain imaging shows that thalamic activity suppression accompanies anesthetic-induced unconsciousness. Therefore, anesthetic-induced unconsciousness may involve direct antagonism of thalamic nicotinic receptors. The authors test this by separately attempting to block or enhance anesthetic-induced loss of righting in rats using intrathalamic microinjections of nicotine or its antagonist. METHODS: Rats were implanted with a cannula aimed at the thalamus or control locations. A week later, loss of righting was induced using sevoflurane (1.4 +/- 0.2%). A dose-parameter study (n = 35) first identified an optimal intrathalamic nicotine dose associated with arousal. Subsequently, this dose was used to pinpoint the thalamic site mediating the arousal response (n = 107). Finally, sevoflurane righting dose and response specificity were assessed after blocking nicotinic channels with intrathalamic mecamylamine pretreatment (n = 8) before nicotine challenge. RESULTS: Nicotine (150 microg/0.5 microl over 1 min) was the optimal arousal dose, because lower doses (75 microg) were ineffective and higher doses (300 microg) often caused seizures. Nicotine temporarily restored righting and mobility in animals when microinjections involved the central medial thalamus (P < 0.0001, chi-square). Righting occurred despite continued sevoflurane administration. Intrathalamic mecamylamine pretreatment did not lower the sevoflurane dose associated with loss of righting, but prevented the nicotine arousal response. CONCLUSIONS: The reversal of unconsciousness found here with intrathalamic microinfusion of nicotine suggests that suppression of the midline thalamic cholinergic arousal system is part of the mechanism by which anesthetics produce unconsciousness.

Memory enhancing effect of low-dose sevoflurane does not occur in basolateral amygdala-lesioned rats.

BACKGROUND: Certain anesthetics might enhance aversive memory at doses around 0.1 minimum alveolar concentration. This issue was investigated in a rat model of learning and memory. In addition, evidence for basolateral amygdala (BLA) involvement in mediating memory enhancement was sought. METHODS: First, the memory-enhancing potential of various anesthetics was determined. Rats underwent single-trial inhibitory avoidance training (0.3 mA shock/1 s) during exposure to air, 0.11% sevoflurane, 0.10% halothane, 0.77% desflurane, or 0.12% isoflurane. Memory was assessed at 24 h. Second, the BLA contribution to sevoflurane memory enhancement was determined. Rats received bilateral excitotoxic N-methyl-D-aspartate (12.5 mg in 0.2 microl per BLA) lesions of the BLA 1 week before training. Memory of lesioned and control rats was compared 24 h after training in air or sevoflurane. RESULTS: Sevoflurane exposure during training significantly enhanced 24-h retention performance for both nonoperated and sham-operated rats (P < 0.005 for both vs. their respective controls). Halothane, but not desflurane or isoflurane, also enhanced retention performance (P < 0.05). However, halothane-induced hyperalgesia during learning clouds interpreting enhanced retention performance solely as a memory consolidation effect. BLA lesions significantly reduced and equalized retention performance for both sevoflurane- and air-exposed animals. Lesions blocked memory enhancement without also causing a generalized inability to learn, because additional training revealed essentially normal task acquisition and 24-h memory. CONCLUSIONS: Sevoflurane enhances aversive memory formation in the rat. The BLA likely contributes to this effect. The risk of aversive memory formation may be enhanced during exposure to low-dose sevoflurane.

Structural brain variation, age, and response time.

Response time (RT) generally slows with aging, but the contribution of structural brain changes to this slowing is unknown. We used voxel-based morphometry (VBM) to determine gray matter (GM) and white matter (WM) brain volumes in 9 middle-aged adults (38-58 years old) and 9 seniors (66-82 years old). We correlated brain volumes with RT assessed in both a simple visual stimulus-response task and a visual continuous recognition memory task. No GM correlations with simple RT were significant; there was one WM correlation in the right fusiform gyrus. In the memory task, faster RT was correlated (p < .05, corrected) with less GM in the globus pallidus, the parahippocampus, and the thalamus for both groups. Several Brodmann areas (BA) differed between the groups such that in each area, less GM was correlated with slower RTs in the middle-aged group but with faster RTs in the senior group (BAs 19, 37, 46, 9, 8, 6, 13, 10, 41, and 7). The results suggest that individual differences in specific brain structure volumes should be considered as potential moderating factors in cognitive brain imaging studies.

General anesthesia and the neural correlates of consciousness.

The neural correlates of consciousness must be identified, but how? Anesthetics can be used as tools to dissect the nervous system. Anesthetics not only allow for the experimental investigation into the conscious-unconscious state transition, but they can also be titrated to subanesthetic doses in order to affect selected components of consciousness such as memory, attention, pain processing, or emotion. A number of basic neuroimaging examinations of various anesthetic agents have now been completed. A common pattern of regional activity suppression is emerging for which the thalamus is identified as a key target of anesthetic effects on consciousness. It has been proposed that a neuronal hyperpolarization block at the level of the thalamus, or thalamocortical and corticocortical reverberant loops, could contribute to anesthetic-induced unconsciousness. However, all anesthetics do not suppress global cerebral metabolism and cause a regionally specific effect on thalamic activity. Ketamine, a so-called dissociative anesthetic agent, increases global cerebral metabolism in humans at doses associated with a loss of consciousness. Nevertheless, it is proposed that those few anesthetics not associated with a global metabolic suppression effect might still have their effects on consciousness mediated at the level of thalamocortical interactions, if such agents scramble the signals associated with normal neuronal network reverberant activity. Functional and effective connectivity are analysis techniques that can be used with neuroimaging to investigate the signal scrambling effects of various anesthetics on network interactions. Whereas network interactions have yet to be investigated with ketamine, a thalamocortical and corticocortical disconnection effect during unconsciousness has been found for both suppressive anesthetic agents and for patients who are in the persistent vegetative state. Furthermore, recovery from a vegetative state is associated with a reconnection of functional connectivity. Taken together these intriguing observations offer strong empirical support that the thalamus and thalamocortical reverberant network loop interactions are at the heart of the neurobiology of consciousness.