Ken Solt

Differential modulation of human N-methyl-D-aspartate receptors by structurally diverse general anesthetics

N-Methyl-d-aspartate (NMDA) receptors have a presumed role in excitatory synaptic transmission and nociceptive pathways. Although previous studies have found that inhaled anesthetics inhibit NMDA receptor-mediated currents at clinically relevant concentrations, the use of different experimental protocols, receptor subtypes, and/or tissue sources confounds quantitative comparisons of the NMDA receptor inhibitory potencies of inhaled anesthetics. In the present study, we sought to fill this void by defining, using the two-electrode voltage-clamp technique, the extent to which diverse clinical and aromatic inhaled anesthetics inhibit the NR1/NR2B subtype of the human NMDA receptor expressed in Xenopus laevis oocytes. At 1 minimum alveolar anesthetic concentration (MAC), anesthetic compounds reversibly inhibited NMDA receptor currents by 12 ± 6% to 74 ± 6%. These results demonstrate that equianesthetic concentrations of inhaled anesthetics can differ considerably in the extent to which they inhibit NMDA receptors. Such differences may be useful for defining the role that this receptor plays in producing the in vivo actions of general anesthetics.

Active Emergence from Propofol General Anesthesia is Induced by Methylphenidate

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 (TCI) of propofol was also tested. In a separate group of rats, a TCI 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 s (95% CI: 598–897 s, n = 6) to 448 s (95% CI: 371–495 s, n = 6). The difference was statistically significant (P = 0.0051). During continuous propofol anesthesia with a median final target plasma concentration of 4.0 &mgr;g/ml (95% CI: 3.2–4.6, n = 6), none of the rats exhibited purposeful movements after injection of normal saline. After methylphenidate, however, all six rats promptly exhibited arousal and had restoration of righting with a median time of 82 s (95% CI: 30–166 s). Spectral analysis of electroencephalogram data demonstrated a shift in peak power from &dgr; (less than 4 Hz) to &thgr; (4–8 Hz) and &bgr; (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.

The Ageing Brain: Age-dependent changes in the electroencephalogram during propofol and sevoflurane general anaesthesia

BACKGROUND Anaesthetic drugs act at sites within the brain that undergo profound changes during typical ageing. We postulated that anaesthesia-induced brain dynamics observed in the EEG change with age. METHODS We analysed the EEG in 155 patients aged 18-90 yr who received propofol (n=60) or sevoflurane (n=95) as the primary anaesthetic. The EEG spectrum and coherence were estimated throughout a 2 min period of stable anaesthetic maintenance. Age-related effects were characterized by analysing power and coherence as a function of age using linear regression and by comparing the power spectrum and coherence in young (18- to 38-yr-old) and elderly (70- to 90-yr-old) patients. RESULTS Power across all frequency bands decreased significantly with age for both propofol and sevoflurane; elderly patients showed EEG oscillations ∼2- to 3-fold smaller in amplitude than younger adults. The qualitative form of the EEG appeared similar regardless of age, showing prominent alpha (8-12 Hz) and slow (0.1-1 Hz) oscillations. However, alpha band dynamics showed specific age-related changes. In elderly compared with young patients, alpha power decreased more than slow power, and alpha coherence and peak frequency were significantly lower. Older patients were more likely to experience burst suppression. CONCLUSIONS These profound age-related changes in the EEG are consistent with known neurobiological and neuroanatomical changes that occur during typical ageing. Commercial EEG-based depth-of-anaesthesia indices do not account for age and are therefore likely to be inaccurate in elderly patients. In contrast, monitoring the unprocessed EEG and its spectrogram can account for age and individual patient characteristics.

Activation of D1 dopamine receptors induces emergence from isoflurane general anesthesia.

Background:A recent study showed that methylphenidate induces emergence from isoflurane anesthesia. Methylphenidate inhibits dopamine and norepinephrine reuptake transporters. The objective of this study was to test the hypothesis that selective dopamine receptor activation induces emergence from isoflurane anesthesia. Methods:In adult rats, we tested the effects of chloro-APB (D1 agonist) and quinpirole (D2 agonist) on time to emergence from isoflurane general anesthesia. We then performed a dose–response study to test for chloro-APB–induced restoration of righting during continuous isoflurane anesthesia. SCH-23390 (D1 antagonist) was used to confirm that the effects induced by chloro-APB are specifically mediated by D1 receptors. In a separate group of animals, spectral analysis was performed on surface electroencephalogram recordings to assess neurophysiologic changes induced by chloro-APB and quinpirole during isoflurane general anesthesia. Results:Chloro-APB decreased median time to emergence from 330 to 50 s. The median difference in time to emergence between the saline control group (n = 6) and the chloro-APB group (n = 6) was 222 s (95% CI: 77–534 s, Mann–Whitney test). This difference was statistically significant (P = 0.0082). During continuous isoflurane anesthesia, chloro-APB dose-dependently restored righting (n = 6) and decreased electroencephalogram &dgr; power (n = 4). These effects were inhibited by pretreatment with SCH-23390. Quinpirole did not restore righting (n = 6) and had no significant effect on the electroencephalogram (n = 4) during continuous isoflurane anesthesia. Conclusions:Activation of D1 receptors by chloro-APB decreases time to emergence from isoflurane anesthesia and produces behavioral and neurophysiologic evidence of arousal during continuous isoflurane anesthesia. These findings suggest that selective activation of a D1 receptor–mediated arousal mechanism is sufficient to induce emergence from isoflurane general anesthesia.

Electrical Stimulation of the Ventral Tegmental Area Induces Reanimation from General Anesthesia

Background:Methylphenidate or a D1 dopamine receptor agonist induces reanimation (active emergence) from general anesthesia. The authors 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 (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 120 µA. 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 ± 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 &dgr; (<4 Hz) to &thgr; (4–8 Hz). In all rats with substantia nigra electrodes, stimulation did not elicit an arousal response or significant electroencephalogram changes. Conclusions:Electrical stimulation of the VTA, but not the substantia nigra, induces reanimation during general anesthesia with isoflurane or propofol. These results are consistent with the hypothesis that dopamine release by VTA neurons, but not substantia nigra neurons, induces reanimation from general anesthesia.

Differential Effects of Serotonin and Dopamine on Human 5-HT3A Receptor Kinetics: Interpretation within an Allosteric Kinetic Model

Serotonin type 3 (5-HT3) receptors are members of the pentameric Cys-loop superfamily of receptors that modulate synaptic neurotransmission. In response to agonist binding and unbinding, members of this superfamily undergo a series of conformational transitions that define their functional properties. In this study, we report the results of electrophysiological studies using rapid solution exchange designed to characterize and compare the actions of the high-efficacy agonist serotonin and the low-efficacy agonist dopamine on human 5-HT3A receptors expressed in human embryonic kidney HEK293 cells. In the case of serotonin, receptor activation rates varied with agonist concentration, and deactivation occurred as a single-exponential process with a rate that was similar to the maximal rate of desensitization. Receptors recovered slowly from long desensitizing pulses of serotonin with a sigmoidal time course. In the case of dopamine, receptor activation rates were independent of agonist concentration, receptor deactivation occurred as a complex process that was significantly faster than the maximal rate of desensitization, and recovery from desensitization occurred more quickly than with 5-HT and its time course was not sigmoidal. We developed an allosteric kinetic model for 5-HT3A receptor activation, deactivation, desensitization, and resensitization. Interpretation of our results within the context of this model indicated that the distinct modulatory actions of serotonin versus dopamine are largely attributable to the vastly different rates with which these two agonists induce channel opening and dissociate from open and desensitized states.

Contrasting roles of the N-methyl-D-aspartate receptor in the production of immobilization by conventional and aromatic anesthetics.

We hypothesized that N-methyl-d-aspartate (NMDA) receptors mediate some or all of the capacity of inhaled anesthetics to prevent movement in the face of noxious stimulation, and that this capacity to prevent movement correlates directly with the in vitro capacity of such anesthetics to block the NMDA receptor. To test this hypothesis, we measured the effect of IV infusion of the NMDA blockers dizocilpine (MK-801) and (R)-4-(3-phosphonopropyl) piperazine-2-carboxylic acid (CPP) to decrease the MAC (the minimum alveolar concentration of anesthetic that prevents movement in 50% of subjects given a noxious stimulation) of 8 conventional anesthetics (cyclopropane, desflurane, enflurane, halothane, isoflurane, nitrous oxide, sevoflurane, and xenon) and 8 aromatic compounds (benzene, fluorobenzene, o-difluorobenzene, p-difluorobenzene, 1,2,4-trifluorobenzene, 1,3,5-trifluorobenzene, pentafluorobenzene, and hexafluorobenzene) and, for comparison, etomidate. We postulated that MK-801 or CPP infusions would decrease MAC in inverse proportion to the in vitro capacity of these anesthetics to block the NMDA receptor. This notion proved correct for the aromatic inhaled anesthetics, but not for the conventional anesthetics. At the greatest infusion of MK-801 (32 &mgr;g · kg–1· min–1) the MACs of conventional anesthetics decreased by 59.4 ± 3.4% (mean ± sd) and at 8 &mgr;g · kg–1· min–1 by 45.5 ± 4.2%, a decrease not significantly different from a 51.4 ± 19.0% decrease produced in the EC50 for etomidate, an anesthetic that acts solely by enhancing &ggr;-amino butyric acid (GABA) receptors. We conclude that some aromatic anesthetics may produce immobility in the face of noxious stimulation by blocking the action of glutamate on NMDA receptors but that conventional inhaled anesthetics do not.

Optogenetic activation of dopamine neurons in the ventral tegmental area induces reanimation from general anesthesia

Significance Although dopamine is known to promote wakefulness, the specific dopamine circuits in the brain that regulate arousal are not clear. Here we report that selective optogenetic stimulation of ventral tegmental area (VTA) dopamine neurons in mice produces a powerful arousal response sufficient to restore conscious behaviors, including the righting reflex, during continuous, steady-state general anesthesia. Although previous studies found that VTA dopamine neurons do not appear to play a central role in regulating sleep–wake transitions, our findings demonstrate that selective stimulation of these neurons is sufficient to induce the transition from an unconscious, anesthetized state to an awake state. These results suggest that VTA DA neurons play a critical role in promoting wakefulness. 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.

Real-time Closed-loop Control in a Rodent Model of Medically Induced Coma Using Burst Suppression

Background:A medically induced coma is an anesthetic state of profound brain inactivation created to treat status epilepticus and to provide cerebral protection after traumatic brain injuries. The authors hypothesized that a closed-loop anesthetic delivery system could automatically and precisely control the electroencephalogram state of burst suppression and efficiently maintain a medically induced coma. Methods:In six rats, the authors implemented a closed-loop anesthetic delivery system for propofol consisting of: a computer-controlled pump infusion, a two-compartment pharmacokinetics model defining propofol’s electroencephalogram effects, the burst-suppression probability algorithm to compute in real time from the electroencephalogram the brain’s burst-suppression state, an online parameter-estimation procedure and a proportional-integral controller. In the control experiment each rat was randomly assigned to one of the six burst-suppression probability target trajectories constructed by permuting the burst-suppression probability levels of 0.4, 0.65, and 0.9 with linear transitions between levels. Results:In each animal the controller maintained approximately 60 min of tight, real-time control of burst suppression by tracking each burst-suppression probability target level for 15 min and two between-level transitions for 5–10 min. The posterior probability that the closed-loop anesthetic delivery system was reliable across all levels was 0.94 (95% CI, 0.77–1.00; n = 18) and that the system was accurate across all levels was 1.00 (95% CI, 0.84–1.00; n = 18). Conclusion:The findings of this study establish the feasibility of using a closed-loop anesthetic delivery systems to achieve in real time reliable and accurate control of burst suppression in rodents and suggest a paradigm to precisely control medically induced coma in patients.

Case records of the Massachusetts General Hospital. Case 40-2012. A 43-year-old woman with cardiorespiratory arrest after a cesarean section.

From the Department of Obstetrics and Gynecology (J.L.E.), the Divisions of Obstetrical Anesthesia (K.S.) and Cardiac Anesthesia (M.G.F.), Department of Anesthesia, Critical Care, and Pain Medicine, and the Department of Surgery (T.E.M.), Massachusetts General Hospital; and the Departments of Obstetrics, Gynecology, and Reproductive Biology (J.L.E.), Anesthesia (K.S., M.G.F.), and Surgery (T.E.M.), Harvard Medical School — both in Boston.