The (un)conscious mouse as a model for human brain functions: key principles of anesthesia and their impact on translational neuroimaging

In recent years, technical and procedural advances have brought functional magnetic resonance imaging (fMRI) to the field of murine neuroscience. Due to its unique capacity to measure functional activity non-invasively, across the entire brain, fMRI allows for the direct comparison of large-scale murine and human brain functions. This opens an avenue for bidirectional translational strategies to address fundamental questions ranging from neurological disorders to the nature of consciousness. The key challenges of murine fMRI are: (1) to generate and maintain functional brain states that approximate those of calm and relaxed human volunteers, while (2) preserving neurovascular coupling and physiological baseline conditions. Low-dose anesthetic protocols are commonly applied in murine functional brain studies to prevent stress and facilitate a calm and relaxed condition among animals. Yet, current mono-anesthesia has been shown to impair neural transmission and hemodynamic integrity. By linking the current state of murine electrophysiology, Ca(2+) imaging and fMRI of anesthetic effects to findings from human studies, this systematic review proposes general principles to design, apply and monitor anesthetic protocols in a more sophisticated way. The further development of balanced multimodal anesthesia, combining two or more drugs with complementary modes of action helps to shape and maintain specific brain states and relevant aspects of murine physiology. Functional connectivity and its dynamic repertoire as assessed by fMRI can be used to make inferences about cortical states and provide additional information about whole-brain functional dynamics. Based on this, a simple and comprehensive functional neurosignature pattern can be determined for use in defining brain states and anesthetic depth in rest and in response to stimuli. Such a signature can be evaluated and shared between labs to indicate the brain state of a mouse during experiments, an important step toward translating findings across species

Consciousness, Anesthesia and Brain Resting State Networks

Consciousness is a great mystery to science. Despite several attempts, none of the current theories have managed to explain how and why it exists. Theories struggle with fundamental philosophical questions, such as the hard problem, that contests how something mental, like consciousness, can be explained by physical phenomena such as neural activity. Modern neuroscientific study of consciousness puts aside this and a few other yet unreachable questions. It focuses on finding the neural correlates of consciousness (NCC) comprising the physical phenomena, which correlate with certain aspects of consciousness. In the NCC studies, consciousness is usually taken to have two aspects: the states of consciousness, encompassing awake, dreaming, and unconscious states, and contents of consciousness, such as an experienced perceptual stimulus. Experimental anesthesia and functional brain imaging are essential tools for the search of the NCC. Anesthesia offers a reliable and reversible method to alter the subject's state of consciousness. The brain function during these altered states of consciousness can be measured with functional imaging methods, such as functional magnetic resonance imaging (fMRI). It measures neuronal activity via a blood oxygen level dependent (BOLD) signal. Functional connectivity analysis of the BOLD-signal can be used to explore the organization of spatially distinct brain areas into functional networks, which are associated with specific cognitive functions such as attention control and emotional regulation. Recently, several studies have shown that changes in functional connectivity between and within these networks are also associated with altered states of consciousness induced by anesthetic drugs. This review will cover the essential questions and methodology of current exploration of neural correlates of states of consciousness, focusing on the resting state networks and the use of fMRI and experimental anesthesia as research tools. Effects of different anesthetics on these networks are also compared. Different anesthetics used in experimental anesthesia have quite distinct pharmacological mechanisms of action, even though the induced brain functional connectivity patterns resemble each other. Propofol-induced unconsciousness is mediated either from corticocortical or thalamocortical disconnection. The nonspecific thalamic nodes, related to arousal and distribution of information, may have an important role in propofol-induced unconsciousness. Dexmedetomidine has similar patterns in connectivity changes as propofol, but the connectivity between deeper brain regions and thalamus remains less affected, possibly explaining the easier arousal from dexmedetomidine-induced unconsciousness. Ketamine increases the overall functional connectivity but disrupts the connectivity in the higher-order networks of the brain inducing “dissociative anesthesia”. The role of thalamic functional connectivity during ketamine anesthesia has not been studied and would be an interesting subject for future studies.Tietoisuuden syntymekanismi on yksi suurimmista ratkaisemattomista mysteereistä. Filosofiset kysymykset, kuten tietoisuuden niin kutsuttu vaikea ongelma ja selityksellinen kuilu, ovat edelleen ajankohtaisia ja ne kyseenalaistavat esimerkiksi pystytäänkö tietoisuuden syntymekanismeja koskaan selittämään tieteellisin keinoin. Nykyinen neurotieteellinen tutkimus pyrkii selvittämään tietoisuuden hermostollisia vastineita, eli kuvaamaan tiettyyn tietoisuuden tilaan tai sisältöön liittyviä aivotoiminnan mekanismeja. Näitä vastineita tutkittaessa käytetään jakoa tietoisuuden tiloihin, joita ovat hereillä olo, unennäkö ja tajuttomuus tai tietoisuuden komponentteihin joita ovat esimerkiksi koettu auditorinen tai visuaalinen ärsyke. Tässä katsausartikkelissa käydään läpi nykyisen tietoisuuden tutkimuksen ja sen tilojen neurologisten vastineiden etsimisen perusteet, keskittyen funktionaaliseen magneettikuvaukseen, lepotilaverkostoihin ja anestesiaan. Artikkelissa myös vertaillaan aikaisempien sellaisten tutkimusten tuloksia, jotka käsittelevät eri anesteettien vaikutuksista lepotilaverkostoihin. Kokeellista anestesiaa ja toiminnallista aivokuvantamista käytetään työkaluina tietoisuuden tiloihin liittyvien aivotoiminnan mekanismien tutkimisessa. Anestesian avulla voidaan luotettavasti ja palautuvasti aiheuttaa muutoksia koehenkilöiden tajunnan tilaan. Samalla tutkimuskysymyksiin etsitään vastauksia kuvantamalla tiettyihin tajunnan tiloihin tai niiden muutoksiin liittyviä aivotoiminnan mekanismeja. Kuvantamismenetelmistä toiminnallisella magneettikuvauksella pysytään mittaamaa hermokudoksen aktiivisuutta veren happipitoisuudesta riippuvalla signaalilla. Vertaamalla aktivoitumista tietyltä alueelta verraten sitä koko aivojen samanaikaiseen aktivoitumiseen, saadaan näkyviin toiminnallisia verkostoja, joilla on omia tehtäviä esimerkiksi kognitiivisessa prosessoinnissa ja tarkkaavaisuuden keskittämisessä. Myös tietoisuuden tasot korreloivat näiden verkostojen sisäisten ja keskinäisten yhteyksien sekä oikea-aikaisen aktivoitumisen kanssa. Kokeellista anestesiaa käytettäessä on huomioitava eri anesteettien erilaiset vaikutusmekanismit, vaikkakin aivokuvantamisella aktivoitumismuutokset muistuttavat toisiaan. Vertailtavissa anesteeteissa esiintyy yhtäläisyyksiä kortikaalisten verkostojen yhteyksien heikkenemiseen ja tietoisuuden häviämiseen liittyen. Propofolin keskeisenä mekanismina on joko yhteyksien katkaiseminen aivokuoren verkostoissa tai talamuksen ja aivokuoren välillä. Deksmedetomidiinin fMRI:llä mitatut vaikutukset muistuttavat paljon propofolia, mutta yhteys talamuksen ja syvempien aivoalueiden välillä säilyy, joka todennäköisesti mahdollistaa nopeamman heräämisen. Ketamiini lisää toiminnallista konnektiivisuutta verkostoissa, kuitenkin samalla hajottaen järjestäytymistä korkeamman asteen verkostoissa. Propofolista poiketen ketamiinin vaikutus toimintaverkostoon on vähäinen. Talamuksen merkitys yhteyksien muutoksissa yleisesti jää vielä epävarmaksi: ovatko muutokset syytä vai seurausta aivokuoren yhteyksien hajoamisesta

Muting, not fragmentation, of functional brain networks under general anesthesia

© 2021 Changes in resting-state functional connectivity (rs-FC) under general anesthesia have been widely studied with the goal of identifying neural signatures of consciousness. This work has commonly revealed an apparent fragmentation of whole-brain network structure during unconsciousness, which has been interpreted as reflecting a break-down in connectivity and a disruption of the brain\u27s ability to integrate information. Here we show, by studying rs-FC under varying depths of isoflurane-induced anesthesia in nonhuman primates, that this apparent fragmentation, rather than reflecting an actual change in network structure, can be simply explained as the result of a global reduction in FC. Specifically, by comparing the actual FC data to surrogate data sets that we derived to test competing hypotheses of how FC changes as a function of dose, we found that increases in whole-brain modularity and the number of network communities – considered hallmarks of fragmentation – are artifacts of constructing FC networks by thresholding based on correlation magnitude. Taken together, our findings suggest that deepening levels of unconsciousness are instead associated with the increasingly muted expression of functional networks, an observation that constrains current interpretations as to how anesthesia-induced FC changes map onto existing neurobiological theories of consciousness

Anaesthesia and PET of the Brain

Although drugs have been used to administer general anaesthesia for more than a century and a half, relatively little was known until recently about the molecular and cellular effects of the anaesthetic agents and the neurobiology of anaesthesia. Positron emission tomography (PET) and single-photon emission computed tomography (SPECT) studies have played a valuable role in improving this knowledge. PET studies using 11C-flumazenil binding have been used to demonstrate that the molecular action of some, but not all, of the current anaesthetic agents is mediated via the GABAA receptor. Using different tracers labelled with 18F, 11C and 15O, PET studies have shown the patterns of changes in cerebral metabolism and blood flow associated with different intravenous and volatile anaesthetic agents. Within classes of volatile agents, there are minor variations in patterns. More profound differences are found between classes of agents. Interestingly, all agents cause alterations in the blood flow and metabolism of the thalamus, providing strong support for the hypothesis that the anaesthetic agents interfere with consciousness by interfering with thalamocortical communication.</p

Intrinsic organization of the anesthetized brain

The neural mechanism of unconsciousness has been a major unsolved question in neuroscience despite its vital role in brain states like coma and anesthesia. The existing literature suggests that neural connections, information integration, and conscious states are closely related. Indeed, alterations in several important neural circuitries and networks during unconscious conditions have been reported. However, how the whole-brain network is topologically reorganized to support different patterns of information transfer during unconscious states remains unknown. Here we directly compared whole-brain neural networks in awake and anesthetized states in rodents. Consistent with our previous report, the awake rat brain was organized in a nontrivial manner and conserved fundamental topological properties in a way similar to the human brain. Strikingly, these topological features were well maintained in the anesthetized brain. Local neural networks in the anesthetized brain were reorganized with altered local network properties. The connectional strength between brain regions was also considerably different between the awake and anesthetized conditions. Interestingly, we found that long-distance connections were not preferentially reduced in the anesthetized condition, arguing against the hypothesis that loss of long-distance connections is characteristic to unconsciousness. These findings collectively show that the integrity of the whole-brain network can be conserved between widely dissimilar physiologic states while local neural networks can flexibly adapt to new conditions. They also illustrate that the governing principles of intrinsic brain organization might represent fundamental characteristics of the healthy brain. With the unique spatial and temporal scales of resting-state fMRI, this study has opened a new avenue for understanding the neural mechanism of (un)consciousness

Disruption of thalamic functional connectivity is a neural correlate of dexmedetomidine-induced unconsciousness

Understanding the neural basis of consciousness is fundamental to neuroscience research. Disruptions in cortico-cortical connectivity have been suggested as a primary mechanism of unconsciousness. By using a novel combination of positron emission tomography and functional magnetic resonance imaging, we studied anesthesia-induced unconsciousness and recovery using the α2-agonist dexmedetomidine. During unconsciousness, cerebral metabolic rate of glucose and cerebral blood flow were preferentially decreased in the thalamus, the Default Mode Network (DMN), and the bilateral Frontoparietal Networks (FPNs). Cortico-cortical functional connectivity within the DMN and FPNs was preserved. However, DMN thalamo-cortical functional connectivity was disrupted. Recovery from this state was associated with sustained reduction in cerebral blood flow and restored DMN thalamo-cortical functional connectivity. We report that loss of thalamo-cortical functional connectivity is sufficient to produce unconsciousness. DOI: http://dx.doi.org/10.7554/eLife.04499.00

General anesthesia, sleep and coma

In the United States, nearly 60,000 patients per day receive general anesthesia for surgery.1 General anesthesia is a drug-induced, reversible condition that includes specific behavioral and physiological traits — unconsciousness, amnesia, analgesia, and akinesia — with concomitant stability of the autonomic, cardiovascular, respiratory, and thermoregulatory systems.2 General anesthesia produces distinct patterns on the electroencephalogram (EEG), the most common of which is a progressive increase in low-frequency, high-amplitude activity as the level of general anesthesia deepens3,4 (Figure 1Figure 1Electroencephalographic (EEG) Patterns during the Awake State, General Anesthesia, and Sleep.). How anesthetic drugs induce and maintain the behavioral states of general anesthesia is an important question in medicine and neuroscience.6 Substantial insights can be gained by considering the relationship of general anesthesia to sleep and to coma. Humans spend approximately one third of their lives asleep. Sleep, a state of decreased arousal that is actively generated by nuclei in the hypothalamus, brain stem, and basal forebrain, is crucial for the maintenance of health.7,8 Normal human sleep cycles between two states — rapid-eye-movement (REM) sleep and non-REM sleep — at approximately 90-minute intervals. REM sleep is characterized by rapid eye movements, dreaming, irregularities of respiration and heart rate, penile and clitoral erection, and airway and skeletal-muscle hypotonia.7 In REM sleep, the EEG shows active high-frequency, low-amplitude rhythms (Figure 1). Non-REM sleep has three distinct EEG stages, with higher-amplitude, lower-frequency rhythms accompanied by waxing and waning muscle tone, decreased body temperature, and decreased heart rate. Coma is a state of profound unresponsiveness, usually the result of a severe brain injury.9 Comatose patients typically lie with eyes closed and cannot be roused to respond appropriately to vigorous stimulation. A comatose patient may grimace, move limbs, and have stereotypical withdrawal responses to painful stimuli yet make no localizing responses or discrete defensive movements. As the coma deepens, the patient's responsiveness even to painful stimuli may diminish or disappear. Although the patterns of EEG activity observed in comatose patients depend on the extent of the brain injury, they frequently resemble the high–amplitude, low-frequency activity seen in patients under general anesthesia10 (Figure 1). General anesthesia is, in fact, a reversible drug-induced coma. Nevertheless, anesthesiologists refer to it as “sleep” to avoid disquieting patients. Unfortunately, anesthesiologists also use the word “sleep” in technical descriptions to refer to unconsciousness induced by anesthetic drugs.11 (For a glossary of terms commonly used in the field of anesthesiology, see the Supplementary Appendix, available with the full text of this article at NEJM.org.) This review discusses the clinical and neurophysiological features of general anesthesia and their relationships to sleep and coma, focusing on the neural mechanisms of unconsciousness induced by selected intravenous anesthetic drugs.Massachusetts General Hospital. Dept. of Anesthesia and Critical Care, and Pain MedicineNational Institutes of Health (NIH) (Director’s Pioneer Award DP1OD003646)University of Michigan. Dept. of AnesthesiologyNational Institutes of Health (U.S.) (grant HL40881)National Institutes of Health (U.S.) (grant HL65272)James S. McDonnell FoundationNational Institutes of Health (U.S.) (grant HD51912

Breakdown of within- and between-network resting state functional magnetic resonance imaging connectivity during propofol-induced loss of consciousness.

BACKGROUND: Mechanisms of anesthesia-induced loss of consciousness remain poorly understood. Resting-state functional magnetic resonance imaging allows investigating whole-brain connectivity changes during pharmacological modulation of the level of consciousness. METHODS: Low-frequency spontaneous blood oxygen level-dependent fluctuations were measured in 19 healthy volunteers during wakefulness, mild sedation, deep sedation with clinical unconsciousness, and subsequent recovery of consciousness. RESULTS: Propofol-induced decrease in consciousness linearly correlates with decreased corticocortical and thalamocortical connectivity in frontoparietal networks (i.e., default- and executive-control networks). Furthermore, during propofol-induced unconsciousness, a negative correlation was identified between thalamic and cortical activity in these networks. Finally, negative correlations between default network and lateral frontoparietal cortices activity, present during wakefulness, decreased proportionally to propofol-induced loss of consciousness. In contrast, connectivity was globally preserved in low-level sensory cortices, (i.e., in auditory and visual networks across sedation stages). This was paired with preserved thalamocortical connectivity in these networks. Rather, waning of consciousness was associated with a loss of cross-modal interactions between visual and auditory networks. CONCLUSIONS: Our results shed light on the functional significance of spontaneous brain activity fluctuations observed in functional magnetic resonance imaging. They suggest that propofol-induced unconsciousness could be linked to a breakdown of cerebral temporal architecture that modifies both within- and between-network connectivity and thus prevents communication between low-level sensory and higher-order frontoparietal cortices, thought to be necessary for perception of external stimuli. They emphasize the importance of thalamocortical connectivity in higher-order cognitive brain networks in the genesis of conscious perception

Functional integration in the cortical neuronal network of conscious and anesthetized animals

General anesthesia consists of amnesia, analgesia, areflexia and unconsciousness. How anesthetics suppress consciousness has been a mystery for more than one and a half centuries. The overall goal of my research has been to determine the neural correlates of anesthetic-induced loss of consciousness. I hypothesized that anesthetics induce unconsciousness by interfering with the functional connectivity of neuronal networks of the brain and consequently, reducing the brain\u27s capacity for information processing. To test this hypothesis, I performed experiments in which neuronal spiking activity was measured with chronically implanted microelectrode arrays in the visual cortex of freely-moving rats during wakefulness and at graded levels of anesthesia produced by the inhalational anesthetic agent desflurane. I then applied linear and non-parametric information-theoretic analyses to quantify the concentration-dependent effect of general anesthetics on spontaneous and visually evoked spike firing activity in rat primary visual cortex. Results suggest that desflurane anesthesia disrupts cortical neuronal integration as measured by monosynaptic connectivity, spike burst coherence and information capacity. This research furthers our understanding of the mechanisms involved with the anesthetic-induced LOC which may facilitate in the development of better anesthetic monitoring devices and the creation of effective anesthetic agents that will be free of unwanted side effects

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 μM) 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 GABA(A )receptor antagonist or chloride channel blocker, suggesting that postsynaptic GABA(A )receptors in VB neurons were involved in the shunting inhibition. GABA(A )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 GABA(A )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 GABA(A )receptor chloride channel-mediated conductance, and such inhibition may contribute to the impaired thalamic responses to sensory stimuli seen during propofol-induced anesthesia