Individual Mesopontine Neurons Implicated in Anesthetic Loss-of-consciousness Employ Separate Ascending Pathways to the Cerebral Cortex

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.

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.].

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.