Subanesthetic Concentrations Research Articles

Differential analgesic effects of subanesthetic concentrations of lidocaine on spontaneous and evoked pain in human painful neuroma: A randomized, double blind study

BackgroundBoth peripheral nerve injury and neuroma pain are the result of changes in sodium channel expression. Lidocaine selectively inhibits the spontaneous ectopic activity by binding to sodium channels. Subanesthetics concentrations of lidocaine are able to produce a differential block of the ectopic discharges, but not propagation of impulses, suppressing differentially the associated neuropathic pain symptoms. The aim of this study was to investigate the differences between the analgesic effects of lidocaine 0.5% and a control group of lidocaine 0.1% on several neuroma related pain modalities. MethodsSixteen patients with neuropathic pain due to painful neuromas caused by nerve injury participated in this randomized, double-blind experiment. The patterns of sensory changes were compared before and after injection of 1ml lidocaine 0.5% and 0.1% close to the neuroma, the sessions being 1–2 weeks apart. Spontaneous and evoked pains were assessed using a visual analogue scale (VAS), quantitative and qualitative sensory testing. The primary end-point measure was defined as the change in pain score measured from baseline until 60min after injection. Assessments of spontaneous pain and evoked pain were done post injection at 15s, 30s, 1min, and at 5-min intervals for the first 30-min post injection and then every 10-min to 1 hr post injection. The assessments of pain were performed between the limbs in the following order: spontaneous pain, then assessment of dynamic mechanical allodynia and then hyperalgesia. ResultsLidocaine dose-dependently reduced spontaneous and evoked pain scores by more than 80% with maximum effects between 1 and 5min for evoked pain and between 3 and 15min for spontaneous pain. While evoked pain normalized rapidly reaching about 50% of the control level 20min after the injection, spontaneous pain levels continue to be lower in comparison with baseline values for more than 60min. When comparing the time course of analgesia between spontaneous and evoked pain, lidocaine-induced a greater reduction of evoked pain, but with shorter duration than spontaneous pain. The differences between evoked pain and spontaneous pain were statistically significant in both groups (lidocaine 0.5% group; p=0.02 and lidocaine 0.1% group; p=0.01). Reproducibility was high for all assessed variables. Surprisingly, both lidocaine concentrations produced a sensory loss within the area with hyperalgesia and allodynia: hypoesthesia occurred earlier and lasted longer with lidocaine 0.5% (between 30s and 5min) in comparison with lidocaine 0.1% (p=0.018). ConclusionDifferential analgesic effects of subanesthetic concentrations of local lidocaine on evoked and spontaneous pain in human neuroma suggest that different mechanisms underlie these two key clinical symptoms. Spontaneous pain and evoked pain need an ongoing peripheral drive and any possible CNS amplification change is temporally closely related to this peripheral input. ImplicationsPainful neuroma represents a clinical model of peripheral neuropathic pain that could lead to a significant step forward in the understanding of pain pathophysiology providing the opportunity to study spontaneous and evoked pain and the underlying mechanisms of neuropathic pain. The proposed model of neuropathic pain allows testing new substances by administration of analgesics directly where the pain is generated.

Equipotent subanesthetic concentrations of sevoflurane and xenon preventing cold-stimulated vocalization of neonatal rats.

The effects of inhaled anesthetics on the developing brain are studied using neonatal rodents exposed to fractions of minimum alveolar concentration (to avoid cardiorespiratory compromise). However, these fractions cannot be assumed to be equipotent. Xenon's anesthetic and neuroprotective properties warrant investigation in these models. Therefore, equipotent, subanesthetic concentrations of inhaled anesthetics are needed. Forty-eight Wistar rats (Charles River Laboratories, Kent, United Kingdom) on postnatal day 9 were randomized to eight concentrations of inhaled anesthetics: isoflurane, sevoflurane, or xenon. Exposure was closely monitored in individual metal-based chambers resting on a 35°C mat to maintain normothermia. A 25°C mat was used to stimulate vocalization and a sound recording made (1 min, 1 to 100 kHz). Rectal temperature or partial pressure of carbon dioxide and pH of mixed arteriovenous blood were measured immediately after the exposure. Concentration-response models were constructed using logistic regression (dependent variable: vocalization and explanatory variable: concentration). The effects of all other explanatory variables were assessed by inserting them individually into the model. The effective inhaled concentrations preventing cold-stimulated vocalization in 50 and 95% of neonatal rats (EiC50 and EiC95) on postnatal day 9 were 0.46 and 0.89% sevoflurane and 20.15 and 34.81% xenon, respectively. The effect on the EiC50 of all other explanatory variables, including duration, was minimal. Stability of EiC50 isoflurane was not achieved over three durations (40, 80, and 120 min exposure). Partial pressure of carbon dioxide and pH in mixed arteriovenous blood appeared normal. The authors report equipotent subanesthetic concentrations of sevoflurane and xenon in neonatal rats with preserved cardiopulmonary function. This may be useful in designing neonatal rodent models of anesthesia.

4‐bromopropofol decreases action potential generation in spinal neurons by inducing a glycine receptor‐mediated tonic conductance

Impaired function of spinal strychnine-sensitive glycine receptors gives rise to chronic pain states and movement disorders. Therefore, increased activity of glycine receptors should help to treat such disorders. Although compounds targeting glycine receptors with a high selectivity are lacking, halogenated analogues of propofol have recently been considered as potential candidates. Therefore we asked whether 4-bromopropofol attenuated the excitability of spinal neurons by promoting glycine receptor-dependent inhibition. The actions of sub-anaesthetic concentrations of propofol and 4-bromopropofol were investigated in spinal tissue cultures prepared from mice. Drug-induced alterations in action potential firing were monitored by extracellular multi-unit recordings. The effects on GABAA and glycine receptor-mediated inhibition were quantified by whole-cell voltage-clamp recordings. Low concentrations of 4-bromopropofol (50 nM) reduced action potential activity of ventral horn neurons by about 30%, compared with sham-treated slices. This effect was completely abolished by strychnine (1 μM). In voltage-clamped neurons, 4-bromopropofol activated glycine receptors, generating a tonic current of 65 ± 10 pA, while GABAA - and glycine receptor-mediated synaptic transmission remained unaffected. The highest glycine levels in the CNS are found in the ventral horn of the spinal cord, a region mediating pain-induced motor reflexes and participating in the control of muscle tone. 4-Bromopropofol may serve as a starting point for the development of non-sedative, non-addictive, muscle relaxants and analgesics to be used to treat low back pain.

Effects of chronic exposure to sub-anesthetic concentrations of sevoflurane on memory function and homeostasis of mice

Objective To evaluate the effects of chronic exposure to sub-anesthetic concentrations of sevoflurane on memory function and homeostasis of mice.Methods Thirty-six healthy male Kunming mice,aged 40 days,weighing 25-30 g,were randomly assigned into 3 groups (n =12 each):control group (group C),0.1％ sevoflurane group (group L) and 0.5 ％ sevoflurane group (group H).The mice inhaled 0.1％ (group L) or 0.5％ sevoflurane (group H) between 18:00-6:00 (the next day) every night for 30 days.Water maze test was performed at 27-30 days of inhalation.Blood samples were collected from the left ventricle for blood gas analysis and for determination of blood electrolytes.Results There was no significant difference in swimming time in Water maze test,number of errors and blood gas analysis and blood electrolytes.Conclusion Chronic exposure to subanesthetic concentrations of sevoflurane has no significant effects on memory function and homeostasis of mice. Key words: Anesthetics, inhalation ; Occupational exposure ; Memory ; Homeostasis

Effects of sub‐anesthetic sevoflurane on cardiorespiratory responses to severe arterial hypoxia in the rabbit (709.4)

Volatile anesthetics may alter reflexes to hypoxia through actions at the carotid body chemoreceptors. The effects of sevoflurane (Sev) in sub‐anesthetic concentrations on integrated chemoreflex control during hypoxia are unknown. The cardiorespiratory responses to end‐tidal Sev (0.18%, 0.37% or 1.86%) were studied in 7 rabbits. Ventilation (VE), tidal volume (VT), respiratory rate (f), heart rate (HR) and arterial pressure (AP) were measured in awake rabbits breathing room air, then 5 minutes of hypoxia (PaO2 < 35 mm Hg), during Sev alone, and Sev plus hypoxia. In awake rabbits hypoxia produced an increase in VE and VT (p<0.05) while f was unchanged. HR fell and AP rose (p<0.05). During normoxia, VT increased and f decreased with all concentrations of Sev, while VE only fell at 0.37% and 1.86% (p<0.05). HR increased with at all concentrations while AP decreased at 0.37% and 1.86% (p<0.05). With hypoxia and Sev, the increase in VT was the same as awake levels at all concentrations associated with a small rise in f (p<0.05). VE increased to levels seen in the awake state at lower concentrations of Sev but was attenuated at 1.86% (p<0.05). The hypoxia induced bradycardia and rise in AP were attenuated by increasing doses of Sev and abolished at 1.86% (p<0.05). Sub‐anesthetic depressant effects of Sev on the cardiorespiratory responses to hypoxia are selective and therefore unlikely to be the result of carotid body inhibition.Grant Funding Source: Supported by John Hunter Hospital Charitable Trust

Effect of Pre-Exposure to Sevoflurane on the Bispectral Index in Women Undergoing Caesarean Delivery Under General Anaesthesia

Editor’s key points † This study assessed the effects of pre-exposure to sevoflurane on the bispectral index (BIS) during induction of anaesthesia for Caesarean section. † Pre-exposure to subanaesthetic concentrations of sevoflurane reduced BIS values in the period before neonatal delivery, and maintained the BIS below 60 in the interval before delivery. † Neither maternal nor Background. Patients undergoing Caesarean delivery under inhalation anaesthesia are at a high risk of awareness, especially in the period before delivery. We assessed the effects of pre-exposure to sevoflurane on the bispectral index (BIS) in the interval before delivery. Methods. Sixty-four patients undergoing elective Caesarean delivery were randomly assigned to receive 1.0‐1.1 vol% (control 1) or 1.2‐1.3 vol% (control 2) end-tidal sevoflurane, or the same concentrations of end-tidal sevoflurane combined with preexposure to 1 vol% sevoflurane for the last 1 min of the preoxygenation period (the preSevo 1 and preSevo 2 groups, respectively). We assessed BIS values, arterial pressure, and heart rate at the time of induction; before intubation; and upon skin incision, uterine incision, and delivery. We also determined the maternal incidence of intraoperative awareness and the neonatal Apgar scores, and conducted umbilical blood gas analysis. Results. At skin incision, BIS values were significantly lower in the preSevo 1 group than in the control 1 group [50 (13) vs 72 (8), P,0.001] and in the preSevo 2 group than in the control 2 group [44 (11) vs 67 (10), P,0.001]. The mean BIS values in the preSevo 1 and 2 groups were maintained below 60 in the period before delivery. No other parameter differed among groups, and no patient exhibited intraoperative awareness.

Effects of Propofol, Sevoflurane, Remifentanil, and (S)-Ketamine in Subanesthetic Concentrations on Visceral and Somatosensory Pain–evoked Potentials

Although electroencephalographic parameters and auditory evoked potentials (AEP) reflect the hypnotic component of anesthesia, there is currently no specific and mechanism-based monitoring tool for anesthesia-induced blockade of nociceptive inputs. The aim of this study was to assess visceral pain-evoked potentials (VPEP) and contact heat-evoked potentials (CHEP) as electroencephalographic indicators of drug-induced changes of visceral and somatosensory pain. Additionally, AEP and electroencephalographic permutation entropy were used to evaluate sedative components of the applied drugs. In a study enrolling 60 volunteers, VPEP, CHEP (amplitude N2-P1), and AEP (latency Nb, amplitude Pa-Nb) were recorded without drug application and at two subanesthetic concentration levels of propofol, sevoflurane, remifentanil, or (s)-ketamine. Drug-induced changes of evoked potentials were analyzed. VPEP were generated by electric stimuli using bipolar electrodes positioned in the distal esophagus. For CHEP, heat pulses were given to the medial aspect of the right forearm using a CHEP stimulator. In addition to AEP, electroencephalographic permutation entropy was used to indicate level of sedation. With increasing concentrations of propofol, sevoflurane, remifentanil, and (s)-ketamine, VPEP and CHEP N2-P1 amplitudes decreased. AEP and electroencephalographic permutation entropy showed neither clinically relevant nor statistically significant suppression of cortical activity during drug application. Decreasing VPEP and CHEP amplitudes under subanesthetic concentrations of propofol, sevoflurane, remifentanil, and (s)-ketamine indicate suppressive drug effects. These effects seem to be specific for analgesia.

Sub-anesthetic concentrations of (R,S)-ketamine metabolites inhibit acetylcholine-evoked currents in α7 nicotinic acetylcholine receptors

The effect of the (R,S)-ketamine metabolites (R,S)-norketamine, (R,S)-dehydronorketamine, (2S,6S)-hydroxynorketamine and (2R,6R)-hydroxynorketamine on the activity of α7 and α3β4 neuronal nicotinic acetylcholine receptors was investigated using patch-clamp techniques. The data indicated that (R,S)-dehydronorketamine inhibited acetylcholine-evoked currents in α7-nicotinic acetylcholine receptor, IC50=55±6nM, and that (2S,6S)-hydroxynorketamine, (2R,6R)-hydroxynorketamine and (R,S)-norketamine also inhibited α7-nicotinic acetylcholine receptor function at concentrations ≤1μM, while (R,S)-ketamine was inactive at these concentrations. The inhibitory effect of (R,S)-dehydronorketamine was voltage-independent and the compound did not competitively displace selective α7-nicotinic acetylcholine receptor ligands [125I]-α-bungarotoxin and [3H]-epibatidine indicating that (R,S)-dehydronorketamine is a negative allosteric modulator of the α7-nicotinic acetylcholine receptor. (R,S)-Ketamine and (R,S)-norketamine inhibited (S)-nicotine-induced whole-cell currents in cells expressing α3β4-nicotinic acetylcholine receptor, IC50 3.1 and 9.1μM, respectively, while (R,S)-dehydronorketamine, (2S,6S)-hydroxynorketamine and (2R,6R)-hydroxynorketamine were weak inhibitors, IC50 >100μM. The binding affinities of (R,S)-dehydronorketamine, (2S,6S)-hydroxynorketamine and (2R,6R)-hydroxynorketamine at the NMDA receptor were also determined using rat brain membranes and the selective NMDA receptor antagonist [3H]-MK-801. The calculated Ki values were 38.95μM for (S)-dehydronorketamine, 21.19μM for (2S,6S)-hydroxynorketamine and>100μM for (2R,6R)-hydroxynorketamine. The results suggest that the inhibitory activity of ketamine metabolites at the α7-nicotinic acetylcholine receptor may contribute to the clinical effect of the drug.

Inhaled analgesia for pain management in labour

Many women would like to have a choice between different types of pain relief during labour, avoiding invasive methods of pain management if they can. One of the options might be inhaled analgesia, which involves the self-administered inhalation of sub-anaesthetic concentrations of agents while the mother remains awake. Most of the agents are easy to administer, can be started in less than a minute and become effective within a similar amount of time. This review makes an important contribution to the evidence base because it's important that women have access to relatively effective and safe analgesia during labour, and that this be available when they need some form of pharmacological pain relief. The review brings together the relevant research to support this option, remembering that even in hospitals with full-time obstetric anaesthesia coverage, no one may be available to place an epidural, offer other effective methods of labour analgesia, or provide a labour-intensive non-pharmacological method to help the woman in pain.[1] Alongside these issues of availability, the more invasive options, such as epidural analgesia, are associated with significant side effects. Our goal was to examine the effects of all modalities of inhaled analgesia on the mother and the baby, for mothers who planned to have a vaginal delivery. We searched widely for randomised trials comparing inhaled analgesia with other inhaled analgesia, placebo or no treatment, or other methods of non-pharmacological pain management in labour; and were able to include 26 studies with nearly 3000 women. We were able to analyse outcomes such as pain intensity, pain relief and side effects like nausea, but, because of a lack of data, we could not analyse other important outcomes, including sense of control, satisfaction with the childbirth experience, mother and baby interaction, breastfeeding, admission to special care baby unit, poor infant outcomes at long-term follow-up and costs. In the comparisons of different types of inhaled analgesia, Flurane derivatives offer slightly better pain relief than nitrous oxide in first stage of labour, leading to a lower pain intensity score and a higher pain relief score. More nausea was also found in the nitrous oxide group, compared with the flurane derivatives group. However, there were major differences between the trials and the findings should also be considered with caution because of the use of a cross-over design for the trials. When nitrous oxide was compared against placebo or no treatment, it was found to offer better pain relief but it also produced more side effects for women such as nausea, vomiting, dizziness and drowsiness. There were no significant differences for any of the outcomes in studies comparing different strengths of inhaled analgesia, different delivery systems or, in one study, comparing inhaled analgesia with Transcutaneous Electrical Nerve Stimulation (TENS). Our conclusion is that inhaled analgesia appears to be effective in reducing pain intensity and in giving pain relief in labour, but that nitrous oxide appears to result in more nausea compared with flurane derivatives and that when compared with no treatment or placebo, nitrous oxide appears to result in even more side effects such as nausea, vomiting, dizziness and drowsiness.

Inhaled analgesia for pain management in labour

Many women would like to have a choice in pain relief during labour and also would like to avoid invasive methods of pain management in labour. Inhaled analgesia during labour involves the self-administered inhalation of sub-anaesthetic concentrations of agents while the mother remains awake and her protective laryngeal reflexes remain intact. Most of the agents are easy to administer, can be started in less than a minute and become effective within a minute. To examine the effects of all modalities of inhaled analgesia on the mother and the newborn for mothers who planned to have a vaginal delivery. We searched the Cochrane Pregnancy and Childbirth Group's Trials Register (31 January 2012), ClinicalTrials.gov, and Current Controlled Trials (2 June 2012), handsearched conference proceedings from the American Society of Clinical Anesthesia (from 1990 to 2011), contacted content experts and trialists and searched reference lists of retrieved studies. Randomised controlled trials comparing inhaled analgesia with other inhaled analgesia or placebo or no treatment or other methods of non-pharmacological pain management in labour. Review authors independently assessed trials for eligibility, methodological quality and extracted all data. Data were double checked for accuracy. Twenty-six studies, randomising 2959 women, were included in this review.Inhaled analgesia versus a different type of inhaled analgesia Pain relief was measured using a Visual Analogue Scale (VAS) from 0 to 100 mm where 100 corresponds to the most relief. Pain intensity was measured using a VAS from 0 to 100 mm, where 0 corresponds to no pain at all and 100 corresponds to the worst pain. The highest score for pain relief is the most positive in contrast to 'pain intensity' in which the higher score is more negative. Flurane derivatives were found to offer better pain relief than nitrous oxide in first stage of labour as measured by a lower pain intensity score (average mean difference (MD) 14.39, 95% confidence interval (CI) 4.41 to 24.37, three studies, 70 women), also a higher pain relief score for flurane derivatives compared with nitrous oxide (average MD -16.32, 95% CI -26.85 to -5.79, two studies, 70 women). Substantial heterogeneity was found in the analyses of pain intensity (P = 0.003) and in the analysis of pain relief (P = 0.002).These findings should be considered with caution because of the questionable design of the included cross-over trials. More nausea was found in the nitrous oxide group compared with the flurane derivatives group (risk ratio (RR) 6.60 95% CI 1.85 to 23.52, two studies, 98 women).Inhaled analgesia versus placebo or no treatment Placebo or no treatment was found to offer less pain relief compared to nitrous oxide (average RR 0.06, 95% CI 0.01 to 0.34, two studies, 310 women; MD -3.50, 95% CI -3.75 to -3.25, one study, 509 women). However, nitrous oxide resulted in more side effects for women such as nausea (RR 43.10, 95% CI 2.63 to 706.74, one study, 509 women), vomiting (RR 9.05, 95% CI 1.18 to 69.32, two studies, 619 women), dizziness (RR 113.98, 95% CI 7.09 to 1833.69, one study, 509 women) and drowsiness (RR 77.59, 95% CI 4.80 to 1254.96, one study, 509 women) when compared with placebo or no treatment.There were no significant differences found for any of the outcomes in the studies comparing one strength versus a different strength of inhaled analgesia, in studies comparing different delivery systems or in the study comparing inhaled analgesia with TENS.Due to lack of data, the following outcomes were not analysed within the review: sense of control; satisfaction with childbirth experience; effect on mother/baby interaction; breastfeeding; admission to special care baby unit; poor infant outcomes at long-term follow-up; or costs. Inhaled analgesia appears to be effective in reducing pain intensity and in giving pain relief in labour. However, substantial heterogeneity was detected for pain intensity. Furthermore, nitrous oxide appears to result in more side effects compared with flurane derivatives. Flurane derivatives result in more drowsiness when compared with nitrous oxide. When inhaled analgesia is compared with no treatment or placebo, nitrous oxide appears to result in even more side effects such as nausea, vomiting, dizziness and drowsiness. There is no evidence for differences for any of the outcomes comparing one strength verus a different strength of inhaled analgesia, comparing different delivery systems or comparing inhaled analgesia with TENS.

Xenon and Sevoflurane Provide Analgesia during Labor and Fetal Brain Protection in a Perinatal Rat Model of Hypoxia-Ischemia

It is not possible to identify all pregnancies at risk of neonatal hypoxic-ischemic encephalopathy (HIE). Many women use some form of analgesia during childbirth and some anesthetic agents have been shown to be neuroprotective when used as analgesics at subanesthetic concentrations. In this study we sought to understand the effects of two anesthetic agents with presumptive analgesic activity and known preconditioning-neuroprotective properties (sevoflurane or xenon), in reducing hypoxia-induced brain damage in a model of intrauterine perinatal asphyxia. The analgesic and neuroprotective effects at subanesthetic levels of sevoflurane (0.35%) or xenon (35%) were tested in a rat model of intrauterine perinatal asphyxia. Analgesic effects were measured by assessing maternal behavior and spinal cord dorsal horn neuronal activation using c-Fos. In separate experiments, intrauterine fetal asphyxia was induced four hours after gas exposure; on post-insult day 3 apoptotic cell death was measured by caspase-3 immunostaining in hippocampal neurons and correlated with the number of viable neurons on postnatal day (PND) 7. A separate cohort of pups was nurtured by a surrogate mother for 50 days when cognitive testing with Morris water maze was performed. Both anesthetic agents provided analgesia as reflected by a reduction in the number of stretching movements and decreased c-Fos expression in the dorsal horn of the spinal cord. Both agents also reduced the number of caspase-3 positive (apoptotic) neurons and increased cell viability in the hippocampus at PND7. These acute histological changes were mirrored by improved cognitive function measured remotely after birth on PND 50 compared to control group. Subanesthetic doses of sevoflurane or xenon provided both analgesia and neuroprotection in this model of intrauterine perinatal asphyxia. These data suggest that anesthetic agents with neuroprotective properties may be effective in preventing HIE and should be tested in clinical trials in the future.

Kinetics of Anesthetic Onset Measured with a Direct Index of Neural Activity

Previous modeling of the kinetics of uptake and elimination of anesthetic drugs from the site of action has used measures derived from the electroencephalogram. Such measures lag the current brain activity because of the time needed to acquire a signal sample and derive the measure. With a direct measure of anesthetic activity, we could model brain uptake more exactly. In volunteers, using a double-blind single-session design, we made repeated measurements using a well-known psychomotor test, the 2 target tapping test, during the washin and washout of 30% nitrous oxide. We also assessed maximal drug effect with a test of cognitive function, the digit symbol substitution test. Concentration at the site of action was modeled from end-tidal measurements, using a simple exponential washin and washout function, with half-times between 0.5 and 3 minutes. Comparisons were made within subjects, using 0 and 5% nitrous oxide. We studied 20 subjects. Nitrous oxide, at 30%, consistently reduced performance of the digit symbol substitution test. Tapping frequency was also reduced, but the effect was less consistent, and only 9 of 20 subjects showed a significant individual reduction in tapping frequency. In these subjects, the relationship between the modeled brain concentration and drug effect was better with a half-time set at 2 minutes, compared with 1.5 or 3 minutes. Given in subanesthetic concentrations, nitrous oxide has rapid onset and offset, consistent with a half-time of 2 minutes. This value is less than the values expected from studies during anesthesia using processed electroencephalogram, but consistent with measures of blood flow to active cerebral tissue in conscious subjects. Studies of performance in conscious subjects may aid further studies of anesthetic kinetics.

Effects of chronic stress on nitrous oxide (N2O)‐induced antinociceptive and anxiolytic effects in mice

Subanesthetic concentrations of N2O are routinely used in clinical dentistry to produce conscious sedation. The aim of this study was to assess the influence of chronic stress on the antinociceptive and anxiolytic effects of N2O. To induce chronic stress, male NIH Swiss mice (20–25 g) were subjected to a modified forced‐swim test four times over 48 min. Thirty min later, they were exposed to 30%, 50% or 70% N2O in O2, and antinociceptive or anxiolytic responsiveness was assessed using the hot plate test or the light/dark exploration box. Non‐stressed control mice responded to N2O with a dose‐dependent antinociceptive effect, but chronically‐stressed, N2O‐exposed mice showed a marked hyperalgesic response. Exposure of non‐stressed control mice to N2O increased both the time spent in the light compartment and the number of intercompartmental transitions in a generally dose‐dependent manner. Chronic stress did not influence either parameter, but chronically‐stressed, N2O‐exposed mice showed increases in the number of transitions but not in time spent in the light compartment. These findings may be of clinical significance in the use of N2O in patients with chronic stress. Further investigation in the influence of stress on the antinociceptive and anxiolytic actions of N2O is warranted. (Supported in part by NIH Grant GM‐77153 and the Allen I. White Distinguished Professorship.)

Is Hydrogen Sulfide-Induced Suspended Animation General Anesthesia?

Hydrogen sulfide (H(2)S) depresses mitochondrial function and thereby metabolic rates in mice, purportedly resulting in a state of "suspended animation." Volatile anesthetics also depress mitochondrial function, an effect that may contribute to their anesthetic properties. In this study, we ask whether H(2)S has general anesthetic properties, and by extension, whether mitochondrial effects underlie the state of anesthesia. We compared loss of righting reflex, electroencephalography, and electromyography in mice exposed to metabolically equipotent concentrations of halothane, isoflurane, sevoflurane, and H(2)S. We also studied combinations of H(2)S and anesthetics to assess additivity. Finally, the long-term effects of H(2)S were assessed by using the Morris water maze behavioral testing 2 to 3 weeks after exposures. Exposure to H(2)S decreases O(2) consumption, CO(2) production, and body temperature similarly to that of the general anesthetics, but fails to produce a loss of righting reflex or muscle atonia at metabolically equivalent concentrations. When combined, H(2)S antagonizes the metabolic effects of isoflurane, but potentiates the isoflurane-induced loss of righting reflex. We found no effect of prior H(2)S exposure on memory or learning. H(2)S (250 ppm), not itself lethal, produced delayed lethality when combined with subanesthetic concentrations of isoflurane. H(2)S cannot be considered a general anesthetic, despite similar metabolic suppression. Metabolic suppression, presumably via mitochondrial actions, is not sufficient to account for the hypnotic or immobilizing components of the anesthetic state. Combinations of H(2)S and isoflurane can be lethal, suggesting extreme care in the combination of these gases in clinical situations.

In Reply

In our recently published article, “Intranasal application of xenon reduces opioid requirement and postoperative pain in patients undergoing major abdominal surgery: A randomized controlled trial,”1we have reported data showing beneficial effects of intranasally applied xenon on intraoperative opioid requirement and postoperative analgesia in patients undergoing abdominal hysterectomy. Beside these main results we have also described the pharmacokinetic of intranasally applied xenon using blood gas analyses. We would like to thank Petrenko and Baba for their interest in our work. We are very pleased to provide additional information regarding our study results and are thankful for the opportunity to extend the discussion about the pharmacokinetic of intranasally applied xenon.In their letter to the editor, Petrenko and Baba question to what extent the xenon concentrations measured in the internal jugular vein of volunteers really reflect the xenon content in cranial blood and target brain tissue under clinical conditions. In addition, they raise the important question whether and in which way lipophilic drugs can reach the brain using a nose-to-brain bypass. Moreover, they discuss the probability that an accumulation of xenon in the anesthesia circuit might be essential for the analgesic effect described in our study.Concentrations of xenon measured in the venous blood of the sagittal sinus of seven anesthetized pigs2or the internal jugular vein (IJV) of two volunteers1reached a steady state of approximately 500 nl/ml after 10 min of intranasal application. There were no reliable differences between xenon concentrations in the venous blood comparing the intracranial sagittal sinus and the brain-draining IJV (fig. 1).A wealth of proof-of-principle studies have already reported nose-to-brain delivery of a range of different drugs in animal models and volunteers. Thus the various pathways that a drug can follow from the nasal cavity to reach the cerebrospinal fluid or the brain tissue have been discussed thoroughly.3,4It is suggested in the literature that a drug administered nasally is able to reach the central nervous system by neural (olfactory, trigeminal) pathways or the bloodstream. Nevertheless, the mechanisms of transport of drugs from the nose to the brain are not yet completely understood. Considering the monoatomic and highly lipophilic nature of xenon as well as the fast wash-in kinetic of nasally applied xenon, we favored a blood-based route of action. However, a neural-related pathway or a combination of different routes cannot be excluded.As stated in our article, the blood gas analyses in both species (pigs, volunteers) were accomplished under comparable conditions.1Blood gas analyses in volunteers served as a preparation of the clinical trial investigating patients undergoing abdominal surgery. To mimic the clinical conditions in the operation theater as closely as possible, the two volunteers were also anesthetized and mechanically ventilated during the blood gas assessment. Two different anesthesia workstations (Draeger Primus [Drägerwerk AG, Lübeck, Germany] for volunteers vs. Draeger Cicero [Drägerwerk AG] for pigs) and different ventilator settings (oxygen flow 300 ml/min for volunteers vs. 1,000 ml/min for pigs) were used in these studies. The concentrations of xenon measured in the blood of mechanically ventilated volunteers and pigs reached a plateau after a few minutes (IJV and sagittal sinus). In addition, under conditions that resemble the clinical setting, peripheral venous (volunteers) and arterial (pigs) xenon concentrations never exceeded 20 nl/ml.2,1Since the measured concentrations of xenon in the blood (IJV and sagittal sinus) of mechanically ventilated study subjects are obviously independent of the ventilator settings, and peripheral xenon concentrations barely reached the quantification limit, we conclude that a relevant accumulation of xenon in the anesthetic circuit is rather unlikely.Given that a relevant portion of xenon was rebreathed, as discussed by Petrenko and Baba, we would expect a continuous increase of xenon concentrations over time and a time-dependent increase of peripheral xenon concentrations. However, this was not the case. We also must consider that because of their defect in xenon tightness (e.g. , caused by silicon tubes), the employed anesthesia workstations are inappropriate for the inhalative application of xenon using an indirect xenon supply at a rate of only 1 l/h.Aside from the discussion about the pharmacokinetics, we would also like to emphasize that there is no evidence for the assumption that clinically relevant effects of xenon are generally limited to blood concentrations in the micromolar range. Note that even approximately 150 nl xenon/ml blood in the IJV are sufficient to suppress pain-related processes of central sensitization.5As mentioned by Petrenko and Baba, every additional application device will complicate the daily anesthetic procedure. Moreover, we have to keep in mind that the maximum amount of xenon uptake using this novel route of application is limited by the available surface of intranasal structures. Therefore we agree with the excellent suggestion that further studies are needed to evaluate the breakeven point between analgesic benefit and economic reality using inhalatively applied xenon in several subanesthetic concentrations.

Getting Oil and Water to Mix

Getting Oil and Water to Mix

NMDA Receptor Hypofunction Phase Couples Independent γ-Oscillations in the Rat Visual Cortex

Hallucinations, a hallmark of psychosis, can be induced by the psychotomimetic N-methyl-D-aspartic acid (NMDA) receptor antagonists, ketamine and phencyclidine (PCP), and are associated with hypersynchronization in the γ-frequency band, but it is unknown how reduced interneuron activation associated with NMDA receptor hypofunction can cause hypersynchronization or distorted perception. Low-frequency γ-oscillations (LFγ) and high-frequency γ-oscillations (HFγ) serve different aspects of perception. In this study, we test whether ketamine and PCP affect the interactions between HFγ and LFγ in the rat visual cortex in vitro. In slices of the rat visual cortex, kainate and carbachol induced LFγ (∼ 34 Hz at 32°C) in layer V and HFγ (∼ 54 Hz) in layer III of the same cortical column. In controls, HFγ and LFγ were independent, and pyramidal neurons recorded in layer III were entrained by HFγ, but not by LFγ. Sub-anesthetic concentrations of ketamine selectively decelerated HFγ by 22 Hz (EC(50)=2.7 μM), to match the frequency of LFγ in layer V. This caused phase coupling of the two γ-oscillations, increased spatial coherence in layer III, and entrained the firing of layer III pyramidal neurons by LFγ in layer V. PCP similarly decelerated HFγ by 22 Hz (EC(50)=0.16 μM), causing cross-layer phase coupling of γ-oscillations. Selective NMDA receptor antagonism, selective NR2B subunit-containing receptor antagonism, and reduced D-serine levels caused a similar selective deceleration of HFγ, whereas increasing NMDA receptor activation through exogenous NMDA, D-serine, or mGluR group 1 agonism selectively accelerated HFγ. The NMDA receptor hypofunction-induced phase coupling of the normally independent γ-generating networks is likely to cause abnormal cross-layer interactions, which may distort perceptions due to aberrant matching of top-down information with bottom-up information. If decelerated HFγ and subsequent cross-layer synchronization also underlie pathological psychosis, acceleration of HFγ could be the target for improved antipsychotic therapy.

GABAA positive modulator and NMDA antagonist-like discriminative stimulus effects of isoflurane vapor in mice

Several neurotransmitter systems have been hypothesized to be involved in the in vivo effects of volatile anesthetics. Drug discrimination may represent a novel procedure to explore the neurochemical systems underlying the sub-anesthetic behavioral effects of these compounds. The purpose of the present study was to examine the contribution of GABA(A) and NMDA receptors to the discriminative stimulus effects of a behaviorally active sub-anesthetic concentration of isoflurane vapor. Sixteen B6SJLF1/J mice were trained to discriminate 10 min of exposure to 6,000 ppm isoflurane vapor from air. Substitution tests were conducted with volatile anesthetics, abused vapors, GABA(A) positive modulators, NMDA antagonists, and nitrous oxide. The volatile anesthetics, enflurane and halothane as well as the abused vapors toluene and 1,1,1-trichloroethane fully substituted for isoflurane. The GABA(A) positive modulators, pentobarbital, midazolam, and zaleplon but not the direct GABA(A) agonist, muscimol, produced high levels of partial substitution for isoflurane. The anticonvulsant, valproic acid fully substituted for isoflurane but a second, tiagabine, did not substitute. The competitive NMDA antagonist, CGS-19755, fully and the non-competitive NMDA antagonist, dizocilpine, partially substituted for isoflurane. The glycine-site NMDA antagonist, L-701,324 did not substitute for isoflurane. Gamma-hydroxybutric acid and nitrous oxide gas also failed to substitute for isoflurane. The discriminative stimulus effects of sub-anesthetic concentrations of isoflurane vapor are shared by other vapor anesthetics and abused inhalants. The discriminative stimulus effects of isoflurane vapor appear to be mediated by both positive allosteric modulation of GABA(A) receptors as well as antagonism of NMDA receptors.

Wasabi and a Volatile Anesthetic

Wasabi and a Volatile Anesthetic

Mechanisms of Inhibition of T-Type Calcium Current in the Reticular Thalamic Neurons by 1-Octanol: Implication of the Protein Kinase C Pathway

Recent studies indicate that T-type calcium channels (T-channels) in the thalamus are cellular targets for general anesthetics. Here, we recorded T-currents and underlying low-threshold calcium spikes from neurons of nucleus reticularis thalami (nRT) in brain slices from young rats and investigated the mechanisms of their modulation by an anesthetic alcohol, 1-octanol. We found that 1-octanol inhibited native T-currents at subanesthetic concentrations with an IC(50) of approximately 4 muM. In contrast, 1-octanol was up to 30-fold less potent in inhibiting recombinant Ca(V)3.3 T-channels heterologously expressed in human embryonic kidney cells. Inhibition of both native and recombinant T-currents was accompanied by a hyperpolarizing shift in steady-state inactivation, indicating that 1-octanol stabilized inactive states of the channel. To explore the mechanisms underlying higher 1-octanol potency in inhibiting native nRT T-currents, we tested the effect of the protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA) and PKC inhibitors. We found that PMA caused a modest increase of T-current, whereas the inactive PMA analog 4alpha-PMA failed to affect T-current in nRT neurons. In contrast, 12-(2-cyanoethyl)-6,7,12,13-tetrahydro-13-methyl-5-oxo-5H-indolo(2,3-a)pyrrolo(3,4-c)-carbazole (Go 6976), an inhibitor of calcium-dependent PKC, decreased baseline T-current amplitude in nRT cells and abolished the effects of subsequently applied 1-octanol. The effects of 1-octanol were also abolished by chelation of intracellular calcium ions with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid. Taken together, these results suggest that inhibition of calcium-dependent PKC signaling is a possible molecular substrate for modulation of T-channels in nRT neurons by 1-octanol.