Assessing changes in volatile general anesthetic sensitivity of mice after local or systemic pharmacological intervention

Microtubule-Stabilizer Epothilone B Delays Anesthetic-Induced Unconsciousness in Rats

Volatile anesthetics are currently believed to cause unconsciousness by acting on one or more molecular targets including neural ion channels, receptors, mitochondria, synaptic proteins, and cytoskeletal proteins. Anesthetic gases including isoflurane bind to cytoskeletal microtubules (MTs) and dampen their quantum optical effects, potentially contributing to causing unconsciousness. This possibility is supported by the finding that taxane chemotherapy consisting of MT-stabilizing drugs reduces the effectiveness of anesthesia during surgery in human cancer patients. In order to experimentally assess the contribution of MTs as functionally relevant targets of volatile anesthetics, we measured latencies to loss of righting reflex (LORR) under 4% isoflurane in male rats injected subcutaneously with vehicle or 0.75 mg/kg of the brain-penetrant MT-stabilizing drug epothilone B (epoB). EpoB-treated rats took an average of 69 s longer to become unconscious as measured by latency to LORR. This was a statistically significant difference corresponding to a standardized mean difference (Cohen's d) of 1.9, indicating a "large" normalized effect size. The effect could not be accounted for by tolerance from repeated exposure to isoflurane. Our results suggest that binding of the anesthetic gas isoflurane to MTs causes unconsciousness and loss of purposeful behavior in rats (and presumably humans and other animals). This finding is predicted by models that posit consciousness as a property of a quantum physical state of neural MTs.

Pharmacodynamics and Pharmacokinetics of HSK3486, a Novel 2,6-Disubstituted Phenol Derivative as a General Anesthetic

Background: The purpose of this study was to characterize the novel sedative/hypnotic agent HSK3486, a 2,6-disubstituted alkylphenol analogue.

Methods: The mechanism of action of HSK3486 was studied in competitive binding assays and whole-cell patch clamp assays. HSK3486 was administered by bolus intravenous injection to dogs and rats, and the loss of righting reflex as well as effects on the cardiovascular and respiratory systems were assessed. The in vitro metabolism of HSK3486 was analyzed by CYP450 genotyping and enzyme inhibition.

Results: HSK3486 competed with t-butylbicycloorthobenzoate (TBOB) and t-butylbicyclophosphorothionate (TBPS) for binding to the gamma-aminobutyric acid type A (GABA_A) receptor. HSK3486 potentiated GABA-evoked chloride currents at lower concentrations while activating GABA_A receptor at higher concentrations. HSK3486 induced hypnosis in rats and dogs, and had a higher therapeutic index than propofol in rats. The hypnotic potency of HSK3486 was approximately 4-5 fold higher than that of propofol. HSK3486 exerted minimal effects on the cardiovascular system.

Conclusions: HSK3486 is a positive allosteric regulator and direct agonist of GABA_A receptor. It has a promising sedative/hypnotic effect and good in vivo pharmacokinetic properties, which justify further studies towards its clinical application.

Comparative effects of dexmedetomidine and propofol on brain and lung damage in experimental acute ischemic stroke

Acute ischemic stroke is associated with pulmonary complications, and often dexmedetomidine and propofol are used to decrease cerebral metabolic rate. However, it is unknown the immunomodulatory actions of dexmedetomidine and propofol on brain and lungs during acute ischemic stroke. The effects of dexmedetomidine and propofol were compared on perilesional brain tissue and lung damage after acute ischemic stroke in rats. Further, the mean amount of both sedatives was directly evaluated on alveolar macrophages and lung endothelial cells primarily extracted 24-h after acute ischemic stroke. In twenty-five Wistar rats, ischemic stroke was induced and after 24-h treated with sodium thiopental (STROKE), dexmedetomidine and propofol. Dexmedetomidine, compared to STROKE, reduced diffuse alveolar damage score [median(interquartile range); 12(7.8–15.3) vs. 19.5(18–24), p = 0.007)], bronchoconstriction index [2.28(2.08–2.36) vs. 2.64(2.53–2.77), p = 0.006], and TNF-α expression (p = 0.0003), while propofol increased VCAM-1 expression compared to STROKE (p = 0.0004). In perilesional brain tissue, dexmedetomidine, compared to STROKE, decreased TNF-α (p = 0.010), while propofol increased VCAM-1 compared to STROKE (p = 0.024). In alveolar macrophages and endothelial cells, dexmedetomidine decreased IL-6 and IL-1β compared to STROKE (p = 0.002, and p = 0.040, respectively), and reduced IL-1β compared to propofol (p = 0.014). Dexmedetomidine, but not propofol, induced brain and lung protection in experimental acute ischemic stroke.

Return of the Righting Reflex Does Not Portend Recovery of Cognitive Function in Anesthetized Rats

As the number of individuals undergoing general anesthesia rises globally, it becomes increasingly important to understand how consciousness and cognition are restored after anesthesia. In rodents, levels of consciousness are traditionally captured by physiological responses such as the return of righting reflex (RORR). However, tracking the recovery of cognitive function is comparatively difficult. Here we use an operant conditioning task, the 5-choice serial reaction time task (5-CSRTT), to measure sustained attention, working memory, and inhibitory control in male and female rats as they recover from the effects of several different clinical anesthetics. In the 5-CSRTT, rats learn to attend to a five-windowed touchscreen for the presentation of a stimulus. Rats are rewarded with food pellets for selecting the correct window within the time limit. During each session we tracked both the proportion of correct (accuracy) and missed (omissions) responses over time. Cognitive recovery trajectories were assessed after isoflurane (2% for 1 h), sevoflurane (3% for 20 min), propofol (10 mg/kg I.V. bolus), ketamine (50 mg/kg I.V. infusion over 10 min), and dexmedetomidine (20 and 35 μg/kg I.V. infusions over 10 min) for up to 3 h following RORR. Rats were classified as having recovered accuracy performance when four of their last five responses were correct, and as having recovered low omission performance when they missed one or fewer of their last five trials. Following isoflurane, sevoflurane, and propofol anesthesia, the majority (63-88%) of rats recovered both accuracy and low omission performance within an hour of RORR. Following ketamine, accuracy performance recovers within 2 h in most (63%) rats, but low omission performance recovers in only a minority (32%) of rats within 3 h. Finally, following either high or low doses of dexmedetomidine, few rats (25-32%) recover accuracy performance, and even fewer (0-13%) recover low omission performance within 3 h. Regardless of the anesthetic, RORR latency is not correlated with 5-CSRTT performance, which suggests that recovery of neurocognitive function cannot be inferred from changes in levels of consciousness. These results demonstrate how operant conditioning tasks can be used to assess real-time recovery of neurocognitive function following different anesthetic regimens.

Inactivation of Prefrontal Cortex Delays Emergence From Sevoflurane Anesthesia

Studies aimed at investigating brain regions involved in arousal state control have been traditionally limited to subcortical structures. In the current study, we tested the hypothesis that inactivation of prefrontal cortex, but not two subregions within parietal cortex—somatosensory barrel field and medial/lateral parietal association cortex—would suppress arousal, as measured by an increase in anesthetic sensitivity. Male and female Sprague Dawley rats were surgically prepared for recording electroencephalogram and bilateral infusion into prefrontal cortex (N = 13), somatosensory barrel field (N = 10), or medial/lateral parietal association cortex (N = 9). After at least 10 days of post-surgical recovery, 156 μM tetrodotoxin or saline was microinjected into one of the cortical sites. Ninety minutes after injection, rats were anesthetized with 2.5% sevoflurane and the time to loss of righting reflex, a surrogate for loss of consciousness, was measured. Sevoflurane was stopped after 45 min and the time to return of righting reflex, a surrogate for return of consciousness, was measured. Tetrodotoxin-mediated inactivation of all three cortical sites decreased (p < 0.05) the time to loss of righting reflex. By contrast, only inactivation of prefrontal cortex, but not somatosensory barrel field or medial/lateral parietal association cortex, increased (p < 0.001) the time to return of righting reflex. Burst suppression ratio was not altered following inactivation of any of the cortical sites, suggesting that there was no global effect due to pharmacologic lesion. These findings demonstrate that prefrontal cortex plays a causal role in emergence from anesthesia and behavioral arousal.

Noninvasive Ultrasound Stimulation of Ventral Tegmental Area Induces Reanimation from General Anaesthesia in Mice

Evidence in animals suggests that deep brain stimulation or optogenetics can be used for recovery from disorders of consciousness (DOC). However, these treatments require invasive procedures. This report presents a noninvasive strategy to stimulate central nervous system neurons selectively for recovery from DOC in mice. Through the delivery of ultrasound energy to the ventral tegmental area, mice were aroused from an unconscious, anaesthetized state in this study, and this process was controlled by adjusting the ultrasound parameters. The mice in the sham group under isoflurane-induced, continuous, steady-state general anaesthesia did not regain their righting reflex. On insonation, the emergence time from inhaled isoflurane anaesthesia decreased (sham: 13.63 ± 0.53 min, ultrasound: 1.5 ± 0.19 min, p < 0.001). Further, the induction time (sham: 12.0 ± 0.6 min, ultrasound: 17.88 ± 0.64 min, p < 0.001) and the concentration for 50% of the maximal effect (EC50) of isoflurane (sham: 0.6%, ultrasound: 0.7%) increased. In addition, ultrasound stimulation reduced the recovery time in mice with traumatic brain injury (sham: 30.38 ± 1.9 min, ultrasound: 7.38 ± 1.02 min, p < 0.01). This noninvasive strategy could be used on demand to promote emergence from DOC and may be a potential treatment for such disorders.

Time-lapse 3D Imaging of Phagocytosis by Mouse Macrophages

Phagocytosis plays a key role in host defense, as well as in tissue development and maintenance, and involves rapid, receptor-mediated rearrangements of the actin cytoskeleton to capture, envelop and engulf large particles. Although phagocytic receptors, downstream signaling pathways, and effectors, such as Rho GTPases, have been identified, the dynamic cytoskeletal remodeling of specific receptor-mediated phagocytic events remain unclear. Four decades ago, two distinct mechanisms of phagocytosis, exemplified by Fcγ receptor (FcγR)- and complement receptor (CR)-mediated phagocytosis, were identified using scanning electron microscopy. Binding of immunoglobulin G (IgG)-opsonized particles to FcγRs triggers the protrusion of thin membrane extensions, which initially form a so-called phagocytic cup around the particle before it becomes completely enclosed and retracted into the cell. In contrast, complement opsonized particles appear to sink into the phagocyte following binding to complement receptors. These two modes of phagocytosis, phagocytic cup formation and sinking in, have become well established in the literature. However, the distinctions between the two modes have become blurred by reports that complement receptor-mediated phagocytosis may induce various membrane protrusions. With the availability of high resolution imaging techniques, phagocytosis assays are required that allow real-time 3D (three dimensional) visualization of how specific phagocytic receptors mediate the uptake of individual particles. More commonly used approaches for the study of phagocytosis, such as end-point assays, miss the opportunity to understand what is happening at the interface of particles and phagocytes. Here we describe phagocytic assays, using time-lapse spinning disk confocal microscopy, that allow 3D imaging of single phagocytic events. In addition, we describe assays to unambiguously image Fcγ receptor- or complement receptor-mediated phagocytosis.

Brainstem Influence on Thalamocortical Oscillations during Anesthesia Emergence

Theories of mechanisms that impair or prevent consciousness during anesthesia that are related to thalamocortical oscillations have been proposed. Many methods of EEG analysis have been proposed as measures of anesthetic effects but only a few have potential to provide measures of those anesthetic effects that are directly related to thalamocortical oscillations. Some of these methods will be explained and demonstrated with examples chosen to provide evidence for or against two of the proposed mechanisms. The first of the two mechanisms to be addressed is the “traveling peak” (Ching et al., 2010), which relates to anesthetic agents synchronizing neural oscillations that occur in subjects who are awake and reducing their frequency from the gamma (25–40 Hz) to the beta range (13–24 Hz) as a state of sedation develops. The mechanism continues to lower the frequency of this oscillation to the alpha (8–12 Hz) range. In the alpha frequency range, responses to sounds and words stop. It has been proposed that the mechanism changes fundamentally at this point and the oscillations are not compatible with consciousness. The second mechanism that will be addressed is a modification of the generally accepted mechanism for the spindle oscillations that occur during natural sleep (Steriade et al., 1993a,b). These two different mechanisms imply two different patterns for changes in the frequency of the thalamocortical oscillations during emergence. The first mechanism implies that the frequency of the oscillations should increase from the alpha range to the beta range during emergence. The “spindle” mechanism implies that the frequency of the oscillation would not increase much beyond the alpha range. Examples of EEG recordings during anesthesia and emergence from anesthesia were found which were consistent with either mechanism alone or both mechanisms at the same time. Neither theory was able to explain all examples. It is possible that both mechanisms can occur and that brainstem activity may influence the characteristics of emergence. The brainstem activity in question may be influenced by nociception and analgesic supplementation. It may be possible to control the path of emergence by controlling brainstem activity with opioids and other agents in order to allow the patient to awaken without going through an excitement phase or delirium at the transition to consciousness.

α2-Adrenergic stimulation of the ventrolateral preoptic nucleus destabilizes the anesthetic state

The sleep-promoting ventrolateral preoptic nucleus (VLPO) shares reciprocal inhibitory inputs with wake-active neuronal nuclei, including the locus ceruleus. Electrophysiologically, sleep-promoting neurons in the VLPO are directly depolarized by the general anesthetic isoflurane and hyperpolarized by norepinephrine, a wake-promoting neurotransmitter. However, the integration of these competing influences on the VLPO, a sleep- and anesthetic-active structure, has yet to be evaluated in either brain slices in vitro or the intact organism. Single-cell multiplex RT-PCR conducted on both isoflurane-activated, putative sleep-promoting VLPO neurons and neighboring, state-indifferent VLPO neurons in mouse brain slices revealed widespread expression of α2A-, α2B- and α2C-adrenergic receptors in both populations. Indeed, both norepinephrine and the highly selective α2 agonist dexmedetomidine each reversed the VLPO depolarization induced by isoflurane in slices in vitro. When microinjected directly into the VLPO of a mouse lightly anesthetized with isoflurane, dexmedetomidine increased behavioral arousal and reduced the depressant effects of isoflurane on barrel cortex somatosensory-evoked potentials but failed to elicit spectral changes in spontaneous EEG. Based on these observations, we conclude that local modulation of α-adrenergic activity in the VLPO destabilizes, but does not fully antagonize, the anesthetic state, thus priming the brain for anesthetic emergence.

Sustained increase in α5GABAA receptor function impairs memory after anesthesia

Many patients who undergo general anesthesia and surgery experience cognitive dysfunction, particularly memory deficits that can persist for days to months. The mechanisms underlying this postoperative cognitive dysfunction in the adult brain remain poorly understood. Depression of brain function during anesthesia is attributed primarily to increased activity of γ-aminobutyric acid type A receptors (GABA(A)Rs), and it is assumed that once the anesthetic drug is eliminated, the activity of GABA(A)Rs rapidly returns to baseline and these receptors no longer impair memory. Here, using a murine model, we found that a single in vivo treatment with the injectable anesthetic etomidate increased a tonic inhibitory current generated by α5 subunit-containing GABA(A)Rs (α5GABA(A)Rs) and cell-surface expression of α5GABA(A)Rs for at least 1 week. The sustained increase in α5GABA(A)R activity impaired memory performance and synaptic plasticity in the hippocampus. Inhibition of α5GABA(A)Rs completely reversed the memory deficits after anesthesia. Similarly, the inhaled anesthetic isoflurane triggered a persistent increase in tonic current and cell-surface expression of α5GABA(A)Rs. Thus, α5GABA(A)R function does not return to baseline after the anesthetic is eliminated, suggesting a mechanism to account for persistent memory deficits after general anesthesia.