Placing a patient in a state of general anesthesia is crucial for safely and humanely performing most surgical and many nonsurgical procedures. How anesthetic drugs create the state of general anesthesia is considered a major mystery of modern medicine. Unconsciousness, induced by altered arousal and/or cognition, is perhaps the most fascinating behavioral state of general anesthesia. We perform a systems neuroscience analysis of the altered arousal states induced by five classes of intravenous anesthetics by relating their behavioral and physiological features to the molecular targets and neural circuits at which these drugs are purported to act. The altered states of arousal are sedation-unconsciousness, sedation-analgesia, dissociative anesthesia, pharmaco-logic non-REM sleep, and neuroleptic anesthesia. Each altered arousal state results from the anesthetic drugs acting at multiple targets in the central nervous system. Our analysis shows that general anesthesia is less mysterious than currently believed.

Dopamine (DA) promotes wakefulness, and DA transporter inhibitors such as dextroamphetamine and methylphenidate are effective for increasing arousal and inducing reanimation, or active emergence from general anesthesia. DA neurons in the ventral tegmental area (VTA) are involved in reward processing, motivation, emotion, reinforcement, and cognition, but their role in regulating wakefulness is less clear. The current study was performed to test the hypothesis that selective optogenetic activation of VTA DA neurons is sufficient to induce arousal from an unconscious, anesthetized state. Floxed-inverse (FLEX)-Channelrhodopsin2 (ChR2) expression was targeted to VTA DA neurons in DA transporter (DAT)-cre mice (ChR2+ group; n = 6). Optical VTA stimulation in ChR2+ mice during continuous, steady-state general anesthesia (CSSGA) with isoflurane produced behavioral and EEG evidence of arousal and restored the righting reflex in 6/6 mice. Pretreatment with the D1 receptor antagonist SCH-23390 before optical VTA stimulation inhibited the arousal responses and restoration of righting in 6/6 ChR2+ mice. In control DAT-cre mice, the VTA was targeted with a viral vector lacking the ChR2 gene (ChR2− group; n = 5). VTA optical stimulation in ChR2− mice did not restore righting or produce EEG changes during isoflurane CSSGA in 5/5 mice. These results provide compelling evidence that selective stimulation of VTA DA neurons is sufficient to induce the transition from an anesthetized, unconscious state to an awake state, suggesting critical involvement in behavioral arousal.anesthesia | ventral tegmental area | dopamine | optogenetics | arousal

Rapid eye movement (REM) sleep is an important component of the natural sleep/wake cycle, yet the mechanisms that regulate REM sleep remain incompletely understood. Cholinergic neurons in the mesopontine tegmentum have been implicated in REM sleep regulation, but lesions of this area have had varying effects on REM sleep. Therefore, this study aimed to clarify the role of cholinergic neurons in the pedunculopontine tegmentum (PPT) and laterodorsal tegmentum (LDT) in REM sleep generation. Selective optogenetic activation of cholinergic neurons in the PPT or LDT during non-REM (NREM) sleep increased the number of REM sleep episodes and did not change REM sleep episode duration. Activation of cholinergic neurons in the PPT or LDT during NREM sleep was sufficient to induce REM sleep.rapid eye movement sleep | acetylcholine | optogenetics | mesopontine tegmentum | mouse R apid eye movement (REM) sleep is tightly regulated, yet the mechanisms that control REM sleep remain incompletely understood. Early pharmacological and unit recording studies suggested that ACh was important for REM sleep regulation (1, 2). For example, injection of cholinergic drugs into the dorsal mesopontine tegmentum reliably induced a state very similar to natural REM sleep in cats (3-6). Unit recordings from the cholinergic areas of the mesopontine tegmentum revealed cells that were active during wakefulness and REM sleep, as well as neurons active only during REM sleep (7-13). Electrical stimulation of the laterodorsal tegmentum (LDT) in cats increased the percentage of time spent in REM sleep (14), and activation of the pedunculopontine tegmentum (PPT) in rats induced wakefulness and REM sleep (15). If cholinergic PPT and LDT neurons are necessary for REM sleep to occur, as the early studies suggest, then lesioning the PPT or LDT should decrease REM sleep. In cats, lesions of the PPT and LDT do disrupt REM sleep (16, 17), but lesions in rodents have had little effect on REM sleep or increased REM sleep (18)(19)(20)(21)(22). Additionally, c-fos studies have found very few cholinergic cells activated under high-REM sleep conditions. When c-fos-positive cholinergic neurons in the PPT and LDT are found to correlate with the percentage of REM sleep, they still account for only a few of the total cholinergic cells in the area (23). Juxtacellular recordings of identified cholinergic neurons in the LDT found these cells had wake and REM active firing profiles, with the majority firing the highest during REM sleep (13). These discrepancies have led to alternative theories of REM sleep regulation, where cholinergic neurons do not play a key role (18, 19, 23, 24 and reviewed in 25, 26).The PPT and LDT are made up of heterogeneous populations of cells, including distinct populations of cholinergic, GABAergic, and glutamatergic neurons (27-29). Many GABAergic neurons are active during REM sleep, as indicated by c-fos (23), and both GABAergic and glutamatergic neurons have been found with maximal firing rates during REM sleep in the LDT and medial PPT (13...

BACKGROUND Methylphenidate or a D1 dopamine receptor agonist induce reanimation (active emergence) from general anesthesia. We tested whether electrical stimulation of dopaminergic nuclei also induces reanimation from general anesthesia. METHODS In adult rats, a bipolar insulated stainless steel electrode was placed in the ventral tegmental area (VTA, n = 5) or substantia nigra (SN, n = 5). After a minimum 7-day recovery period, the isoflurane dose sufficient to maintain loss of righting was established. Electrical stimulation was initiated and increased in intensity every 3 min to a maximum of 120μA. If stimulation restored the righting reflex, an additional experiment was performed at least 3 days later during continuous propofol anesthesia. Histological analysis was conducted to identify the location of the electrode tip. In separate experiments, stimulation was performed in the prone position during general anesthesia with isoflurane or propofol, and the electroencephalogram was recorded. RESULTS To maintain loss of righting, the dose of isoflurane was 0.9% ± 0.1 vol%, and the target plasma dose of propofol was 4.4 μg/ml ± 1.1 μg/ml (mean ± SD). In all rats with VTA electrodes, electrical stimulation induced a graded arousal response including righting that increased with current intensity. VTA stimulation induced a shift in electroencephalogram peak power from δ (<4 Hz) to θ (4–8 Hz). In all rats with SN electrodes, stimulation did not elicit an arousal response or significant electroencephalogram changes. CONCLUSIONS Electrical stimulation of the VTA, but not the SN, induces reanimation during general anesthesia with isoflurane or propofol. These results are consistent with the hypothesis that dopamine release by VTA, but not SN, neurons induces reanimation from general anesthesia.

During prolonged intervals of wakefulness brain adenosine levels rise within the basal forebrain and cortex. The view that adenosine promotes sleep is supported by the corollary that N-methylated xanthines such as caffeine increase brain and behavioral arousal by blocking adenosine receptors. The four subtypes of adenosine receptors are distributed heterogeneously throughout the brain, yet the neurotransmitter systems and brain regions through which adenosine receptor blockade causes arousal are incompletely understood. This study tested the hypothesis that adenosine A1 and A2A receptors in the prefrontal cortex contribute to the regulation of behavioral and cortical arousal. Dependent measures included acetylcholine (ACh) release in the prefrontal cortex, cortical electroencephalographic (EEG) power, and time to waking after anesthesia. Sleep and wakefulness were also quantified after microinjecting an adenosine A1 receptor antagonist into the prefrontal cortex. The results showed that adenosine A1 and A2A receptors in the prefrontal cortex modulate cortical ACh release, behavioral arousal, EEG delta power, and sleep. Additional dual microdialysis studies revealed that ACh release in the pontine reticular formation is significantly altered by dialysis delivery of adenosine receptor agonists and antagonists to the prefrontal cortex. These data, and early brain transection studies demonstrating that the forebrain is not needed for sleep cycle generation, suggest that the prefrontal cortex modulates EEG and behavioral arousal via descending input to the pontine brainstem. The results provide novel evidence that adenosine A1 receptors within the prefrontal cortex comprise part of a descending system that inhibits wakefulness.

BACKGROUND A recent study showed that methylphenidate induces emergence from isoflurane general anesthesia. Isoflurane and propofol are general anesthetics that may have distinct molecular mechanisms of action. The objective of this study was to test the hypothesis that methylphenidate actively induces emergence from propofol general anesthesia. METHODS Using adult rats, the effect of methylphenidate on time to emergence after a single bolus of propofol was determined. The ability of methylphenidate to restore righting during a continuous target controlled infusion of propofol was also tested. In a separate group of rats, a target controlled infusion of propofol was established and spectral analysis was performed on electroencephalogram recordings taken before and after methylphenidate administration. RESULTS Methylphenidate decreased median time to emergence after a single dose of propofol from 735 seconds (95% CI: 598 to 897 seconds, n=6) to 448 seconds (95% CI: 371 to 495 seconds, n=6). The difference was statistically significant (p = 0.0051). During continuous propofol anesthesia with a median final target plasma concentration of 4.0 μg/ml (95%CI: 3.2 to 4.6, n=6), none of the rats exhibited purposeful movements after injection of normal saline. After methylphenidate, however, all 6 rats promptly exhibited arousal and had restoration of righting with a median time of 82 seconds (95% CI: 30 to 166 seconds). Spectral analysis of electroencephalogram data demonstrated a shift in peak power from delta (<4 Hz) to theta (4–8 Hz) and beta (12–30 Hz) after administration of methylphenidate, indicating arousal in 4/4 rats. CONCLUSIONS Methylphenidate decreases time to emergence after a single dose of propofol, and induces emergence during continuous propofol anesthesia in rats. Further study is warranted to test the hypothesis that methylphenidate induces emergence from propofol general anesthesia in humans.

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The periaqueductal gray (PAG) is a significant modulator of both analgesic and fear behaviors in both humans and rodents, but the underlying circuitry responsible for these two phenotypes is incompletely understood. Importantly, it is not known if there is a way to produce analgesia without anxiety by targeting the PAG, as modulation of glutamate or GABA neurons in this area initiates both antinociceptive and anxiogenic behavior. While dopamine (DA) neurons in the ventrolateral PAG (vlPAG)/dorsal raphe display a supraspinal antinociceptive effect, their influence on anxiety and fear are unknown. Using DAT-cre and Vglut2-cre male mice, we introduced designer receptors exclusively activated by designer drugs (DREADD) to DA and glutamate neurons within the vlPAG using viral-mediated delivery and found that levels of analgesia were significant and quantitatively similar when DA and glutamate neurons were selectively stimulated. Activation of glutamatergic neurons, however, reliably produced higher indices of anxiety, with increased freezing time and more time spent in the safety of a dark enclosure. In contrast, animals in which PAG/dorsal raphe DA neurons were stimulated failed to show fear behaviors. DA-mediated antinociception was inhibitable by haloperidol and was sufficient to prevent persistent inflammatory pain induced by carrageenan. In summary, only activation of DA neurons in the PAG/dorsal raphe produced profound analgesia without signs of anxiety, indicating that PAG/dorsal raphe DA neurons are an important target involved in analgesia that may lead to new treatments for pain.