Reduced Sensitivity to Anesthetic Agents upon Lesioning the Mesopontine Tegmental Anesthesia Area in Rats Depends on Anesthetic Type

Striatal neurones expressing D1 dopamine receptors modulate consciousness in sevoflurane but not propofol anaesthesia in mice

BACKGROUND

Sevoflurane and propofol are the most widely used inhaled and i.v. general anaesthetics, respectively. The mechanisms by which sevoflurane and propofol induce loss of consciousness (LOC) remain unclear. Recent studies implicate the brain dopaminergic circuit in anaesthetic-induced LOC and the cortical-striatal-thalamic-cortical loop in decoding consciousness. We investigated the contribution of the dorsal striatum, which is a critical interface between the dopaminergic circuit and the cortical-striatal-thalamic-cortical loop, in sevoflurane and propofol anaesthesia.

METHODS

Electroencephalography and electromyography recordings and righting reflex tests were used to determine LOC and recovery of consciousness (ROC). The activity of D1 dopamine receptor (D1R)-expressing neurones in the dorsal striatum was monitored using fibre photometry, and regulated using optogenetic and chemogenetic methods in D1R-Cre mice.

RESULTS

Population activities of striatal D1R neurones began to decrease before LOC and gradually returned after ROC. During sevoflurane anaesthesia, optogenetic activation of striatal D1R neurones induced ROC at cortical and behavioural levels in steady-state anaesthesia and promoted cortical activation in deep burst suppression anaesthesia. Chemogenetic inhibition of striatal D1R neurones accelerated induction (from 242.0 [46.1] to 194.0 [26.9] s; P=0.010) and delayed emergence (from 93.5 [21.2] to 133.5 [33.9] s; P=0.005), whereas chemogenetic activation of these neurones accelerated emergence (from 107 [23.7] to 81.3 [16.1] s; P=0.011). However, neither optogenetic nor chemogenetic manipulation of striatal D1R neurones had any effects on propofol anaesthesia.

CONCLUSIONS

Striatal D1R neurones modulate the state of consciousness in sevoflurane anaesthesia, but not in propofol anaesthesia.

Molecular and cellular targets of GABAergic anesthetics in the mesopontine tegmentum that enable pain-free surgery

ABSTRACT

The mesopontine tegmental anesthesia area (MPTA) is a focal brainstem locus which, when exposed to GABAergic agents, induces brain-state transitioning from wakefulness to unconsciousness. Correspondingly, MPTA lesions render animals relatively insensitive to GABAergic anesthetics delivered systemically. Using chemogenetics, we recently identified a neuronal subpopulation within the MPTA whose excitation induces this same pro-anesthetic effect. However, very few of these "effector-neurons" express synaptic γ2-containing GABAA receptor isoforms and none express extrasynaptic δ-subunit containing receptors, suggesting that they are not the direct cellular target of GABAergic agents. Here we used pharmacological tools in rats to define the molecular target(s) of GABAergics in the MPTA. GABA microinjected into the MPTA at nanomolar concentrations, selective for GABAAδ-Rs, proved to be pro-anesthetic as was blocking GABA reuptake. Likewise, low-concentration gaboxadol/THIP, also selective for GABAAδ-Rs, was effective, whereas benzodiazepines and zolpidem, which selectively target GABAAγ2-Rs, were not. The GABAergic anesthetics pentobarbital and propofol proved pro-anesthetic when applied to the MPTA at the low concentrations present in the brain after systemic dosing. Glycinergic agonists which are inhibitory, but infective on GABAAδ-Rs, and other non-GABAergic agonists tested, were at most only marginally effective. We conclude that GABAAδ-Rs are the primary molecular target of GABAergic anesthetics in the MPTA. Immunolabeling revealed that this GABAA-R isoform is expressed exclusively by a distinct subpopulation of MPTA "δ-cells" that reside in close apposition to effector neurons. This suggests that during wakefulness, δ-cells serve as inhibitory interneurons which, when silenced by GABAergic agents, disinhibit (excite) the effector-neurons, triggering transition to unconsciousness.

Neurosteroids foster sedation by engaging tonic GABAA-Rs within the mesopontine tegmental anesthesia area (MPTA)

Neurosteroids are endogenous molecules with anxiolytic, anticonvulsant, sleep-promoting and sedative effects. They are biosynthesized de novo within the brain, among other tissues, and are thought to act primarily as positive allosteric modulators of high-affinity extrasynaptic GABAAδ-receptors. The location of action of neurosteroids in the brain, however, remains unknown. We have demonstrated that GABAergic anesthetics act within the brainstem mesopontine tegmental anesthesia area (MPTA) to induce and maintain anesthetic loss-of-consciousness. Here we asked whether endogenous and synthetic neurosteroids might also act in the MPTA to induce their suppressive effects. Direct exposure of the MPTA to the endogenous neurosteroids pregnenolone and progesterone, their metabolites testosterone, allopregnanolone and 3α5α-THDOC, and the synthetic neurosteroids ganaxolone and alphaxalone, was found to be pro-anesthetic. Although we cannot rule out additional sites of action, results of this study suggest that the suppressive effects of neurosteroids are due, at least in part, to actions within the MPTA, presumably by recruitment of dedicated neuronal circuitry. This undermines the usual presumption that neurosteroids, like other sedatives, endogenous somnogens and anesthetics, act by nonspecific global distribution.

Ventrolateral periaqueductal gray GABAergic neurons promote arousal of sevoflurane anesthesia through cortico-midbrain circuit

The mechanism of general anesthesia remains elusive. The ventrolateral periaqueductal gray (vlPAG) in the midbrain regulates sleep and awake states. However, the role of vlPAG and its circuits in anesthesia is unclear. We utilized opto/chemogenetics, righting reflex, and electroencephalographic recording to assess consciousness changes. We employed fiber photometry to measure the activity of neurons and neurotransmitters. As a result, photometry recording showed that the activity of GABA neurons in vlPAG decreased during sevoflurane anesthesia and was reactivated after anesthesia. Activating GABAergic neurons in vlPAG promoted arousal during anesthesia, while inhibiting them delayed this process. Furthermore, medial prefrontal cortex (mPFC) to vlPAG pyramidal neurons projections and vlPAG to ventral tegmental area (VTA) GABAergic projections played a prominent role in the anesthesia-awake transition. GABA neurotransmitter activity of VTA synchronized with mPFC-vlPAG pyramidal neuron projections. Therefore, the cortico-midbrain circuits centered on vlPAG GABAergic neurons exert an arousal-promoting effect during sevoflurane anesthesia.

Rapid thalamocortical network switching mediated by cortical synchronization underlies propofol-induced EEG signatures: a biophysical model

Propofol-mediated unconsciousness elicits strong alpha/low-beta and slow oscillations in the electroencephalogram (EEG) of patients. As anesthetic dose increases, the EEG signal changes in ways that give clues to the level of unconsciousness; the network mechanisms of these changes are only partially understood. Here, we construct a biophysical thalamocortical network involving brain stem influences that reproduces transitions in dynamics seen in the EEG involving the evolution of the power and frequency of alpha/low-beta and slow rhythm, as well as their interactions. Our model suggests that propofol engages thalamic spindle and cortical sleep mechanisms to elicit persistent alpha/low-beta and slow rhythms, respectively. The thalamocortical network fluctuates between two mutually exclusive states on the timescale of seconds. One state is characterized by continuous alpha/low-beta-frequency spiking in thalamus (C-state), whereas in the other, thalamic alpha spiking is interrupted by periods of co-occurring thalamic and cortical silence (I-state). In the I-state, alpha colocalizes to the peak of the slow oscillation; in the C-state, there is a variable relationship between an alpha/beta rhythm and the slow oscillation. The C-state predominates near loss of consciousness; with increasing dose, the proportion of time spent in the I-state increases, recapitulating EEG phenomenology. Cortical synchrony drives the switch to the I-state by changing the nature of the thalamocortical feedback. Brain stem influence on the strength of thalamocortical feedback mediates the amount of cortical synchrony. Our model implicates loss of low-beta, cortical synchrony, and coordinated thalamocortical silent periods as contributing to the unconscious state.NEW & NOTEWORTHY GABAergic anesthetics induce alpha/low-beta and slow oscillations in the EEG, which interact in dose-dependent ways. We constructed a thalamocortical model to investigate how these interdependent oscillations change with propofol dose. We find two dynamic states of thalamocortical coordination, which change on the timescale of seconds and dose-dependently mirror known changes in EEG. Thalamocortical feedback determines the oscillatory coupling and power seen in each state, and this is primarily driven by cortical synchrony and brain stem neuromodulation.

From molecule to oblivion: dedicated brain circuitry underlies anesthetic loss of consciousness permitting pain-free surgery

The canonical view of how general anesthetics induce loss-of-consciousness (LOC) permitting pain-free surgery posits that anesthetic molecules, distributed throughout the CNS, suppress neural activity globally to levels at which the cerebral cortex can no longer sustain conscious experience. We support an alternative view that LOC, in the context of GABAergic anesthesia at least, results from anesthetic exposure of a small number of neurons in a focal brainstem nucleus, the mesopontine tegmental anesthesia area (MPTA). The various sub-components of anesthesia, in turn, are effected in distant locations, driven by dedicated axonal pathways. This proposal is based on the observations that microinjection of infinitesimal amounts of GABAergic agents into the MPTA, and only there, rapidly induces LOC, and that lesioning the MPTA renders animals relatively insensitive to these agents delivered systemically. Recently, using chemogenetics, we identified a subpopulation of MPTA "effector-neurons" which, when excited (not inhibited), induce anesthesia. These neurons contribute to well-defined ascending and descending axonal pathways each of which accesses a target region associated with a key anesthetic endpoint: atonia, anti-nociception, amnesia and LOC (by electroencephalographic criteria). Interestingly, the effector-neurons do not themselves express GABA_A-receptors. Rather, the target receptors reside on a separate sub-population of presumed inhibitory interneurons. These are thought to excite the effectors by disinhibition, thus triggering anesthetic LOC.

Understanding the Neural Mechanisms of General Anesthesia from Interaction with Sleep-Wake State: A Decade of Discovery

The development of cutting-edge techniques to study specific brain regions and neural circuits that regulate sleep-wake brain states and general anesthesia (GA), has increased our understanding of these states that exhibit similar neurophysiologic traits. This review summarizes current knowledge focusing on cell subtypes and neural circuits that control wakefulness, rapid eye movement (REM) sleep, non-REM sleep, and GA. We also review novel insights into their interactions and raise unresolved questions and challenges in this field. Comparisons of the overlapping neural substrates of sleep-wake and GA regulation will help us to understand sleep-wake transitions and how anesthetics cause reversible loss of consciousness. SIGNIFICANCE STATEMENT: General anesthesia (GA), sharing numerous neurophysiologic traits with the process of natural sleep, is administered to millions of surgical patients annually. In the past decade, studies exploring the neural mechanisms underlying sleep-wake and GA have advanced our understanding of their interactions and how anesthetics cause reversible loss of consciousness. Pharmacotherapies targeting the neural substrates associated with sleep-wake and GA regulations have significance for clinical practice in GA and sleep medicine.

Anesthetic loss of consciousness induced by chemogenetic excitation of mesopontine effector neurons

Although general anesthesia is normally induced by systemic dosing, an anesthetic state can be induced in rodents by microinjecting minute quantities of GABAergic agents into the brainstem mesopontine tegmental anesthesia area (MPTA). Correspondingly, lesions to the MPTA render rats relatively insensitive to standard anesthetic doses delivered systemically. Using a chemogenetic approach we have identified and characterized a small subpopulation of neurons restricted to the MPTA which, when excited, render the animal anesthetic by sensorimotor (immobility) and electroencephalographic (EEG) criteria. These "effector-neurons" do not express GABA_Aδ-Rs, the likely target of GABAergic anesthetics. Rather, we report a distinct sub-population of nearby MPTA neurons which do. During anesthetic induction these likely excite the effector-neurons by disinhibition. Within the effector population ~ 70% appear to be glutamatergic, ~30% GABAergic and ~ 40% glycinergic. Most are projection neurons that send ascending or descending axons to distant targets associated with the individual functional components of general anesthesia: atonia, analgesia, amnesia, and loss-of-consciousness.

Anesthesia in mice activates discrete populations of neurons throughout the brain

The brain undergoes rapid, dramatic, and reversible transitioning between states of wakefulness and unconsciousness during natural sleep and in pathological conditions such as hypoxia, hypotension, and concussion. Transitioning can also be induced pharmacologically using general anesthetic agents. The effect is selective. Mobility, sensory perception, memory formation, and awareness are lost while numerous housekeeping functions persist. How is selective transitioning accomplished? Classically a handful of brainstem and diencephalic "arousal nuclei" have been implicated in driving brain-state transitions on the grounds that their net activity systematically varies with brain state. Here we used transgenic targeted recombination in active populations mice to label neurons active during wakefulness with one reporter and neurons active during pentobarbital-induced general anesthesia with a second, contrasting reporter. We found 'wake-on' and 'anesthesia-on' neurons in widely distributed regions-of-interest, but rarely encountered neurons labeled with both reporters. Nearly all labeled neurons were either wake-on or anesthesia-on. Thus, anesthesia-on neurons are not unique to the few nuclei discovered to date whose activity appears to increase during anesthesia. Rather neuronal populations selectively active during anesthesia are located throughout the brain where they likely play a causative role in transitioning between wakefulness and anesthesia. The widespread neuronal suppression reported in prior comparisons of the awake and anesthetized brain in animal models and noninvasive imaging in humans reflects only net differences. It misses the ubiquitous presence of neurons whose activity increases during anesthesia. The balance in recruitment of anesthesia-on versus wake-on neuronal populations throughout the brain may be a key driver of regional and global vigilance states. [Correction added on September 22, 2021, after first online publication: Due to a typesetting error, the abstract text was cut off. This has been corrected now.].

Identifying c-fos Expression as a Strategy to Investigate the Actions of General Anesthetics on the Central Nervous System

Although general anesthetics have been used in the clinic for more than 170 years, the ways in which they induce amnesia, unconsciousness, analgesia, and immobility remain elusive. Modulations of various neural nuclei and circuits are involved in the actions of general anesthetics. The expression of the immediate-early gene c-fos and its nuclear product, c-fos protein, can be induced by neuronal depolarization; therefore, c-fos staining is commonly used to identify the activated neurons during sleep and/or wakefulness, as well as in various physiological conditions in the central nervous system. Identifying c-fos expression is also a direct and convenient method to explore the effects of general anesthetics on the activity of neural nuclei and circuits. Using c-fos staining, general anesthetics have been found to interact with sleep- and wakefulness-promoting systems throughout the brain, which may explain their ability to induce unconsciousness and emergence from general anesthesia. This review summarizes the actions of general anesthetics on neural nuclei and circuits based on a c-fos expression.

A nodal point for brain-state transitions: the mesopontine tegmental anesthesia area (MPTA) in mice

The mesopontine tegmental anesthesia area (MPTA) was identified in rats as a singular brainstem locus at which microinjection of minute quantities of GABAergic agents rapidly and reversibly induces loss-of-consciousness and a state of general anesthesia, while lesioning renders animals insensitive to anesthetics at normal systemic doses. Obtaining similar results in mice has been challenging, however, slowing research progress on how anesthetics trigger brain-state transitions. We have identified roadblocks that impeded translation from rat to mouse and tentatively located the MPTA equivalent in this second species. We describe here a series of modifications to the rat protocol that allowed us to document pro-anesthetic changes in mice following localized stereotactic delivery of minute quantities (20 nL) of the GABA_A-receptor agonist muscimol into the brainstem mesopontine tegmentum. The optimal locus identified proved to be homologous to the MPTA in rats, and local neuronal populations in rats and mice were similar in size and shape. This outcome should facilitate application of the many innovative gene-based methodologies available primarily in mice to the study of how activity in brainstem MPTA neurons brings about anesthetic loss-of-consciousness and permits pain-free surgery.

The Neural Circuits Underlying General Anesthesia and Sleep

General anesthesia is characterized by loss of consciousness, amnesia, analgesia, and immobility. Important molecular targets of general anesthetics have been identified, but the neural circuits underlying the discrete end points of general anesthesia remain incompletely understood. General anesthesia and natural sleep share the common feature of reversible unconsciousness, and recent developments in neuroscience have enabled elegant studies that investigate the brain nuclei and neural circuits underlying this important end point. A common approach to measure cortical activity across the brain is electroencephalogram (EEG), which can reflect local neuronal activity as well as connectivity among brain regions. The EEG oscillations observed during general anesthesia depend greatly on the anesthetic agent as well as dosing, and only some resemble those observed during sleep. For example, the EEG oscillations during dexmedetomidine sedation are similar to those of stage 2 nonrapid eye movement (NREM) sleep, but high doses of propofol and ether anesthetics produce burst suppression, a pattern that is never observed during natural sleep. Sleep is primarily driven by withdrawal of subcortical excitation to the cortex, but anesthetics can directly act at both subcortical and cortical targets. While some anesthetics appear to activate specific sleep-active regions to induce unconsciousness, not all sleep-active regions play a significant role in anesthesia. Anesthetics also inhibit cortical neurons, and it is likely that each class of anesthetic drugs produces a distinct combination of subcortical and cortical effects that lead to unconsciousness. Conversely, arousal circuits that promote wakefulness are involved in anesthetic emergence and activating them can induce emergence and accelerate recovery of consciousness. Modern neuroscience techniques that enable the manipulation of specific neural circuits have led to new insights into the neural circuitry underlying general anesthesia and sleep. In the coming years, we will continue to better understand the mechanisms that generate these distinct states of reversible unconsciousness.

Paradoxical anesthesia: Sleep-like EEG during anesthesia induced by mesopontine microinjection of GABAergic agents

General anesthetic agents are thought to induce loss-of-consciousness (LOC) and enable pain-free surgery by acting on the endogenous brain circuitry responsible for sleep-wake cycling. In clinical use, the entire CNS is exposed to anesthetic molecules with LOC and amnesia usually attributed to synaptic suppression in the cerebral cortex and immobility and analgesia to agent action in the spinal cord and brainstem. This model of patch-wise suppression has been challenged, however, by the observation that all functional components of anesthesia can be induced by focal delivery of minute quantities of GABAergic agonists to the brainstem mesopontine tegmental anesthesia area (MPTA). We compared spectral features of the cortical electroencephalogram (EEG) in rats during systemic anesthesia and anesthesia induced by MPTA microinjection. Systemic administration of (GABAergic) pentobarbital yielded the sustained, δ-band dominant EEG signature familiar in clinical anesthesia. In contrast, anesthesia induced by MPTA microinjection (pentobarbital or muscimol) featured epochs of δ-band EEG alternating with the wake-like EEG, the pattern typical of natural non-rapid-eye-movement (NREM) and REM sleep. The rats were not sleeping, however, as they remained immobile, atonic and unresponsive to noxious pinch. Recalling the paradoxical wake-like quality the EEG during REM sleep, we refer to this state as "paradoxical anesthesia". GABAergic anesthetics appear to co-opt both cortical and spinal components of the sleep network via dedicated axonal pathways driven by MPTA neurons. Direct drug exposure of cortical and spinal neurons is not necessary, and is probably responsible for off-target side-effects of systemic administration including monotonous δ-band EEG, hypothermia and respiratory depression. SIGNIFICANCE STATEMENT: The concept that GABAergic general anesthetic agents induce loss-of-consciousness by substituting for an endogenous neurotransmitter, thereby co-opting neural circuitry responsible for sleep-wake transitions, has gained considerable traction. However, the electroencephalographic (EEG) signatures of sleep and anesthesia differ fundamentally. We show that when the anesthetic state is generated by focal delivery of GABAergics into the mesopontine tegmental anesthesia area (MPTA) the resulting EEG repeatedly transitions between delta-wave-dominant and wake-like patterns much as in REM-NREM sleep. This suggests that systemic (clinical) anesthetic delivery, which indiscriminately floods the entire cerebrum with powerful inhibitory agents, obscures the sleep-like EEG signature associated with the less adulterated form of anesthesia obtained when the drugs are applied selectively to loci where the effective neurotransmitter substitution actually occurs.

Activation of GABAergic Neurons in the Rostromedial Tegmental Nucleus and Other Brainstem Regions Promotes Sedation and Facilitates Sevoflurane Anesthesia in Mice

Many general anesthetics potentiate gamma-aminobutyric acid (GABA) A receptors but their neuroanatomic sites of action are less clear. GABAergic neurons in the rostromedial tegmental nucleus (RMTg) send inhibitory projections to multiple arousal-promoting nuclei, but the role of these neurons in modulating consciousness is unknown. In this study, designer receptors exclusively activated by designer drugs (DREADDs) were targeted to RMTg GABAergic neurons of Vgat-ires-Cre mice. DREADDs expression was found in the RMTg and other brainstem regions. Activation of these neurons decreased movement and exploratory behavior, impaired motor coordination, induced electroencephalogram (EEG) oscillations resembling nonrapid eye movement (NREM) sleep without loss of righting and reduced the dose requirement for sevoflurane-induced unconsciousness. These results suggest that GABAergic neurons in the RMTg and other brainstem regions promote sedation and facilitate sevoflurane-induced unconsciousness.

Manipulating Neural Circuits in Anesthesia Research

The neural circuits underlying the distinct endpoints that define general anesthesia remain incompletely understood. It is becoming increasingly evident, however, that distinct pathways in the brain that mediate arousal and pain are involved in various endpoints of general anesthesia. To critically evaluate this growing body of literature, familiarity with modern tools and techniques used to study neural circuits is essential. This Readers' Toolbox article describes four such techniques: (1) electrical stimulation, (2) local pharmacology, (3) optogenetics, and (4) chemogenetics. Each technique is explained, including the advantages, disadvantages, and other issues that must be considered when interpreting experimental results. Examples are provided of studies that probe mechanisms of anesthesia using each technique. This information will aid researchers and clinicians alike in interpreting the literature and in evaluating the utility of these techniques in their own research programs.

Patterns of neural activity in the mouse brain: Wakefulness vs. General anesthesia

In light of the general shift from rats to mice as the leading rodent model in neuroscience research we used c-Fos expression as a tool to survey brain regions in the mouse in which neural activity differs between the states of wakefulness and pentobarbital-induced general anesthesia. The aim was to complement prior surveys carried out in rats. In addition to a broad qualitative review, 28 specific regions of interest (ROIs) were evaluated quantitatively. Nearly all ROIs in the cerebral cortex showed suppressed activity during anesthesia. Subcortically, however, some ROIs showed suppression, some showed little change, and some showed increased activity. The overall picture was similar to the rat. Special attention was devoted to ROIs significantly activated during anesthesia, as such loci might actively drive the transition to anesthetic unconsciousness rather than responding passively to inhbitory agents distributed globally (the "wet blanket" hypothesis). Twelve such "anesthesia-on" ROIs were identified: the paraventricular hypothalamic nucleus, supraoptic nucleus, tuberomamillary nucleus, lateral habenular nucleus, dentate gyrus, nucleus raphe pallidus, central amygdaloid nucleus, perifornical lateral hypothalamus, ventro-lateral preoptic area, lateral septum, paraventricular thalamic nucleus and zona incerta. The same primary anti-FOS antibody was used in all mice, but two alternative reporter systems were employed: ABC-diaminobenzidine and the currently more popular AlexaFluor488. Fluorescence tagging revealed far fewer FOS-immunoreactive neurons, sounding an alert that the reporter system chosen can have major effects on results obtained.