The effect of the alpha 2-agonist dexmedetomidine and the N-methyl-D-aspartate antagonist S(+)-ketamine on the expression of apoptosis-regulating proteins after incomplete cerebral ischemia and reperfusion in rats

In this study, we investigated whether the neuroprotection previously seen with dexmedetomidine or S(+)-ketamine involves regulation of proapoptotic (Bax and p53) and antiapoptotic (Bcl-2 and Mdm-2) proteins. Rats were anesthetized with isoflurane. After surgical preparation of isoflurane was discontinued, animals were randomly assigned to receive fentanyl and nitrous oxide (N(2)O)/oxygen plus 100 microg/kg of dexmedetomidine intraperitoneally 30 min before ischemia (n = 8), 1 mg x kg(-1) x min(-1) of S(+)-ketamine and oxygen/air (n = 8), or fentanyl and N(2)O/oxygen (n = 8; control group). In all three treatment groups, incomplete cerebral ischemia (30 min) was induced by unilateral carotid artery occlusion and hemorrhagic hypotension to a mean arterial blood pressure of 30-35 mm Hg. Four hours after the start of reperfusion, the brains were removed, and the expression of apoptosis-regulating proteins was determined by using immunofluorescence and Western blot analysis. The results were compared with sham-operated animals (n = 8). After cerebral ischemia/reperfusion, the relative protein concentration of Bax was increased by 110% in control animals compared with the dexmedetomidine- and S(+)-ketamine-treated rats and by 140% compared with the sham-operated animals. In animals treated with dexmedetomidine, the expression of Bcl-2 and Mdm-2 was larger compared with control (68% and 210%, respectively) or sham-operated (110% and 180%, respectively) animals. Therefore, it is possible that the neuroprotective properties of dexmedetomidine and S(+)-ketamine seen in previous studies involve ultra-early modulation of the balance between pro- and antiapoptotic proteins.

Effects of xenon on mesenteric blood flow

BACKGROUND AND OBJECTIVE

The effects of xenon on mesenteric vascular resistance have not been investigated. Because human beings anaesthetized with xenon show good cardiovascular stability, we believed that the agent would have little or no effect on vascular resistance in the splanchnic bed. We determined the effects of different inhaled xenon concentrations on mesenteric blood flow and mesenteric oxygen consumption in pigs sedated with intravenous propofol.

METHODS

Twenty-three minipigs were instrumented with transit time flow probes around the pulmonary and superior mesenteric arteries as well as with pulmonary artery and portal venous catheters. A 14 h recovery was allowed followed by recordings of baseline values. Xenon was then randomly administered in 0.30, 0.50, and 0.70 end-tidal fractions.

RESULTS

The administration of xenon resulted in an 8% (not dose dependent) decrease in mean arterial pressure (from 99 +/- 15 to 91 +/- 19 mmHg; P < 0.05), a 20% decrease in calculated systemic oxygen consumption (from 0.23 +/- 0.07 to 0.19 +/- 0.04L min(-1); P < 0.01), a 20% reduction in mesenteric oxygen delivery (from 41 +/- 12 to 33 +/- 11 mL min; P < 0.001), a 37% reduction in mesentericmetabolic rate of oxygen (from 11.3 +/- 3.6 to 7.1 +/- 3.2 mL min(-1); P < 0.01) and an 8% decrease in mesenteric artery blood flow (0.22 +/- 0.07 to 0.20 +/- 0.07 L min(-1); P < 0.05) in a dose-dependent fashion. Heart rate, cardiac output, systemic vascular resistance, mesenteric vascular resistance, mesenteric oxygen extraction fraction and portal lactate concentration were not significantly altered by xenon.

CONCLUSIONS

Xenon inhalation in the propofol-sedated pig had no measurable effects on mesenteric vascular resistance. This finding may partly explain the well-known cardiovascular stability observed in patients anaesthetized with xenon. Although mesenteric artery blood flow and mesenteric oxygen delivery decreased during xenon administration, unchanged mesenteric oxygen extraction fraction and portal lactate suggest that metabolic regulation of the splanchnic circulation remained unaltered.

GABA(A) receptor activation and open-channel block by volatile anaesthetics: a new principle of receptor modulation?

The rapid application of solutions containing the volatile anaesthetics isoflurane or sevoflurane induced inward currents in human embryonic kidney (HEK293) cells carrying rat recombinant alpha(1)beta(2)gamma(2L) GABA(A) receptor assemblies. The responses evoked by the anaesthetics applied via a fast delivery system were recorded using the patch-clamp technique in the whole-cell mode. The anaesthetics induced a fast inward current which was followed by a prominent tail current upon the rapid withdrawal of the agent. These currents were simulated using a kinetic scheme embodying two agonist-like binding steps required for receptor activation, and one binding step by which the anaesthetic induces an open-channel block. According to this model of a biphasic receptor modulation, the open-channel block delays the ion flux through the ligand-gated receptors and, thus, prolongs the overall duration of the current response. Open-channel blocks might also be operative in other ligand-gated ion channels to modulate synaptic strength.

Differential effects of isoflurane on excitatory and inhibitory synaptic inputs to thalamic neurones in vivo

BACKGROUND

Mechanosensory thalamocortical relay neurones (TCNs) receive glutamatergic excitatory input and are subjected to gamma-aminobutyric acid (GABA)Aergic inhibitory input. This study assessed the effects of an increase in concentration of isoflurane on thalamic excitatory and inhibitory mechanisms.

METHODS

TCNs (n = 15) of the thalamic ventral posteromedial nucleus responding to mechanical stimulation of whiskers were investigated in rats anaesthetized with end-tidal concentrations of isoflurane of approximately 0.9% (ISOlow, baseline) and approximately 1.9% (ISOhigh). Response activity induced by controlled vibratory movement of single whiskers was recorded before, during and after iontophoretic administration of the GABAA receptor antagonist bicuculline to the vicinity of the recorded neurone.

RESULTS

The increase in concentration of isoflurane induced a suppression of vibratory responses to 14 (4)% [mean (SEM)] of baseline activity. Blockade of GABAA receptors by bicuculline during ISOlow and ISOhigh caused increases in response activity to 259 (32)% and 116 (25)% of baseline activity, respectively. The increase in isoflurane concentration enhanced overall inhibitory inputs by 102 (38)%, whilst overall excitatory inputs were reduced by 54 (7)%.

CONCLUSIONS

These data suggest that doubling the concentration of isoflurane doubles the strength of GABAAergic inhibition and decreases the excitatory drive of TCNs by approximately 50%. The isoflurane-induced enhancement of GABAAergic inhibition led to a blockade of thalamocortical information transfer which was not accomplished by the effects of isoflurane on glutamatergic synaptic transmission alone. Thus, it appears that, with respect to transmission of information in the thalamus, the most prominent action of isoflurane is an enhancement of GABAAergic synpatic inhibition, and that effects on glutamatergic neurotransmission may contribute to a lesser extent.

Prediction of movement following noxious stimulation during 1 minimum alveolar anesthetic concentration isoflurane/nitrous oxide anesthesia by means of middle latency auditory evoked responses

This paper investigates the applicability of generalized dynamic neural networks for the design of a two-valued anesthetic depth indicator during isoflurane/nitrous oxide anesthesia. The indicator construction is based on the processing of middle latency auditory evoked responses (MLAER) in combination with the observation of the patient's movement reaction to skin incision. The framework of generalized dynamic neural networks does not require any data preprocessing, visual data inspection or subjective feature extraction. The study is based on a data set of 106 patients scheduled for elective surgery under isoflurane/nitrous oxide anesthesia. The processing of the measured MLAER is performed by a recurrent neural network that transforms the MLAER signals into signals having a very uncomplex structure. The evaluation of these signals is self-evident, and yields to a simple threshold classifier. Using only evoked potentials before the pain stimulus, the patient's reaction could be predicted with a probability of 81.5%. The MLAER is closely associated to the patient's reaction to skin incision following noxious stimulation during 1 minimum alveolar anesthetic concentration isoflurane/nitrous oxide anesthesia. In combination with other parameters, MLAER could contribute to an objective and trustworthy movement prediction to noxious stimulation.

Capillary leak syndrome after cardiopulmonary bypass in elective, uncomplicated coronary artery bypass grafting operations: does it exist?

OBJECTIVE

Operations coupled with cardiopulmonary bypass may provoke a systemic inflammatory response, and it has been suggested that this responses causes capillary leakage of proteins, edema formation, and even organ failure. However, capillary leak syndrome is mainly a clinical diagnosis and has not been verified as yet by actual demonstration of protein leakage from the circulation. We have therefore measured the disappearance of labeled plasma protein before and after cardiopulmonary bypass.

METHODS

Sixteen patients scheduled for elective coronary artery bypass grafting were enrolled in a prospective controlled study. The cardiopulmonary bypass circuit was primed with crystalloids only. Tumor necrosis factor alpha, interleukin 6, interleukin 8, anaphylatoxin C3a, and terminal complement complex C5b9 levels were determined before, during, and 3 hours after cardiopulmonary bypass. The transvascular escape rate of plasma protein from the intravascular compartment was assessed by measuring the disappearance of intravenously injected Evans blue dye before and during the third hour after cardiopulmonary bypass.

RESULTS

A significant inflammatory response could be demonstrated by means of the 5 measured mediators after bypass. The maximal increase, as compared with the baseline value, was found for interleukin 6 (36-fold). The transvascular escape rate of Evans blue dye was similar before and after bypass (7.6 +/- 0.6%/h vs 7.3 +/- 0.6%/h).

CONCLUSIONS

The above data confirm the systemic inflammatory response induced by cardiopulmonary bypass. Contrary to expectations, the transvascular escape rate of Evans blue dye did not change when comparing values before and after bypass. The data do not support the concept of increased protein leakage in the exchange vessels after bypass. We were unable to demonstrate a capillary leak syndrome.

Long-term depression in the basolateral amygdala of the mouse involves the activation of interneurons

Long-term depression (LTD) in the basolateral amygdala, following low frequency stimulation (1 Hz/900 pulses) of the lateral amygdala, was studied in an in vitro slice preparation of 2-3 weeks and 2-4 months old mice. Whole-cell patch-clamp recordings of neurons, visualized by means of infrared videomicroscopy, and extracellular field potential recordings were performed. Loading single neurons with the calcium chelator BAPTA (30 mM) did not reduce the excitatory postsynaptic currents following low frequency stimulation. However, buffering presynaptic calcium with BAPTA-AM, and application of the specific Ca2+/calmodulin-stimulated protein kinase II antagonist KN-62 (1-[N,O-bis(5-isoquinoline sulfonyl)-N-methyl-L-tyrosyl]-4-phenylpiperizine), blocked low frequency stimulation-induced LTD. The induction of LTD was reduced by the competitive N-methyl-D-aspartate receptor antagonist D-(-)-2-amino-5-phosphonopentanoic acid (50 microM), and blocked by the metabotropic glutamate receptor antagonist (-)-amino-4-carboxy-methyl-phenylacetic acid (1 mM), and by 5-hydroxytryptamine (5-HT; 30 microM) via the activation of 5-HT(1A) receptors. Also blocking GABA(A) receptor-mediated synaptic transmission with bicuculline (10 microM) or picrotoxin (20 microM) reduced the induction of LTD. Visually and electrophysiologically identified interneurons in slices from 2 weeks old mice, expressed in contrast to adult mice (2-4 months), pronounced LTD. Principal neurons showed only weak LTD after low frequency stimulation.A synopsis of these findings suggests a pivotal role of GABAergic interneurons and serotonergic afferents in the induction of LTD in the basolateral nucleus of the amygdala.

Effect of the alpha2-agonist dexmedetomidine on cerebral neurotransmitter concentrations during cerebral ischemia in rats

BACKGROUND

This study investigates whether neuroprotection seen with dexmedetomidine is associated with suppression of peripheral or central sympathetic tone.

METHODS

Thirty fasted male Sprague-Dawley rats were intubated and ventilated with isoflurane and N2O/O2 (fraction of inspired oxygen = 0.33). Catheters were inserted into the right femoral artery and vein and into the right jugular vein. Cerebral blood flow was measured using laser Doppler flowmetry. Bilateral microdialysis probes were placed into the cortex and the dorsal hippocampus. At the end of preparation, the administration of isoflurane was replaced by fentanyl (bolus: 10 microg/kg; infusion: 25 microg x kg(-1) x h(-1)). Animals were randomly assigned to one of the following groups: group 1 (n = 10): control animals; group 2 (n = 10): 100 microg/kg dexmedetomidine administered intraperitoneally 30 min before ischemia; group 3 (n = 10): sham-operated rats. Ischemia (30 min) was produced by unilateral carotid artery occlusion plus hemorrhagic hypotension to a mean arterial blood pressure of 30-35 mmHg to reduce ipsilateral cerebral blood flow by 70%. Pericranial temperature, arterial blood gases, and pH were maintained constant. Cerebral catecholamine and glutamate concentrations and plasma catecholamine concentrations were analyzed using high-performance liquid chromatography.

RESULTS

During ischemia, dexmedetomidine suppressed circulating norepinephrine concentrations by 95% compared with control animals. In contrast, brain norepinephrine and glutamate concentrations were increased irrespective of dexmedetomidine infusion before ischemia.

CONCLUSIONS

The current data show that the increase of circulating catecholamine concentrations during cerebral ischemia was suppressed with dexmedetomidine. In contrast, dexmedetomidine does not suppress elevation in brain norepinephrine and glutamate concentration associated with cerebral ischemia. This suggests that the neuroprotective effects of dexmedetomidine are not related to inhibition of presynaptic norepinephrine or glutamate release in the brain.

Increased responsiveness of cortical neurons in contrast to thalamic neurons during isoflurane-induced EEG bursts in rats

The neuronal mechanisms underlying the electroencephalographic (EEG) burst-suppression pattern are not yet understood, however, they are generally attributed to interactions within thalamocortical networks. In contrast, we report that the sensory cortex and the thalamus are disconnected, with thalamic sensory processing being unaffected by cortical EEG bursts. We studied the activity of single neurons of the somatosensory thalamocortical system in rats during burst-suppression EEG induced by the volatile anesthetic, isoflurane. In neurons of the thalamic ventrobasal complex, the discharge rate in response to tactile stimulation of their receptive fields did not differ significantly during EEG bursts and isoelectric periods. In contrast, in neurons of the primary somatosensory cortex, the response magnitude was significantly greater during EEG bursts as compared with isoelectric periods (mean increase to 293%). The results suggest that the profound suppression of cortical sensory information processing by isoflurane is suspended during EEG burst-induced elevated cortical excitation.

Bispectral Index (BIS) may not predict awareness reaction to intubation in surgical patients

Bispectral Index (BIS) has been marketed as a measure of the hypnotic component of anesthesia and is recommended as a guide for the administration of hypnotic drugs during anesthesia. BIS values between 40 and 60 are recommended for surgery under general anesthesia. This study investigates whether a BIS baseline between 50 and 60 prevents awareness reaction to endotracheal intubation. After approval by the university's Ethics Committee, 20 consenting patients were enrolled in the study. Midazolam (0.1 mg/kg PO) was given 30 minutes before induction. Anesthesia was induced with alfentanil (10 mcg/kg, followed by 2 mcg/kg(-1) x min(-1)) and propofol infusion was adjusted to a BIS target level between 50 and 60. Propofol infusion rate was maintained constant for 5 minutes with constant BIS. Prior to intubation, patients were tested in one-minute intervals for awareness using Tunstall's isolated forearm technique. Three minutes after endotracheal intubation, the study period ended and surgery was performed. After intubation, 8 of 20 patients showed an awareness reaction, squeezing the investigator's hand in response to a command. No patient had recall. Comparison of patients with and without awareness reaction revealed no differences in BIS before or after intubation. This study shows that a BIS value between 50 and 60 prior to intubation is inadequate to prevent an awareness reaction to endotracheal intubation during propofol/alfentanil anesthesia. Because BIS cannot differentiate between patients with and without awareness reaction, its value as a monitor for awareness and a measurement of the hypnotic component of anesthesia must be questioned.

S(+)-ketamine/propofol maintain dynamic cerebrovascular autoregulation in humans

PURPOSE

This study investigates the effects of S(+)-ketamine and propofol in comparison to sevoflurane on dynamic cerebrovascular autoregulation in humans.

METHODS

Twenty-four patients were randomly assigned to one of the following anesthetic protocols: group I (n=12): 2.5 mg.kg(-1)*hr(-1) S(+)-ketamine, 1.5-2.5 microg*mL(-1) propofol-target plasma concentration; group II (n=12): 2.0 MAC (4.0 %) sevoflurane. Patients were intubated and ventilated with O(2)/air (PaO(2)=0.33). Following 40 min of equilibration dynamic cerebrovascular autoregulation was measured and expressed as the autoregulatory index (ARI), describing the duration of cerebral hemodynamic recovery in relation to changes in mean arterial blood pressure.

STATISTICS

Mann-Whitney U test (statistical significance was assumed when P <0.05).

RESULTS

Dynamic cerebrovascular autoregulation was intact in all patients with S(+)-ketamine/propofol anesthesia as indicated by an ARI of 5.4 +/- 1.1. In contrast, dynamic cerebrovascular autoregulation was significantly delayed with 2.0 MAC sevoflurane (ARI=2.6 +/- 0.7) CONCLUSION: Dynamic cerebrovascular autoregulation is maintained with S(+)-ketamine/propofol-based total iv anesthesia. In contrast, 2.0 MAC sevoflurane delayed dynamic cerebrovascular autoregulation. This supports the use of S(+)-ketamine in combination with propofol in neurosurgical patients based on its neuroprotective potential along with maintained cerebrovascular physiology.

Wavelet analysis of middle latency auditory evoked responses: calculation of an index for detection of awareness during propofol administration

BACKGROUND

Middle latency auditory evoked responses (MLAER) as a measure of depth of sedation are critically dependent on data quality and the analysis technique used. Manual peak labeling is subject to observer bias. This study investigated whether a user-independent index based on wavelet transform can be derived to discriminate between awake and unresponsive states during propofol sedation.

METHODS

After obtaining ethics committee approval and written informed consent, 13 volunteers and 40 patients were studied. In all subjects, propofol was titrated to loss of response to verbal command. The volunteers were allowed to recover, then propofol was titrated again to the same end point, and subjects were finally allowed to recover. From three MLAER waveforms at each stage, latencies and amplitudes of peaks Pa and Nb were measured manually. In addition, wavelet transform for analysis of MLAER was applied. Wavelet transform gives both frequency and time information by calculation of coefficients related to different frequency contents of the signal. Three coefficients of the so-called wavelet detail level 4 were transformed into a single index (Db3d4) using logistic regression analysis, which was also used for calculation of indices for Pa, Nb, and Pa/Nb latencies. Prediction probabilities for discrimination between awake and unresponsive states were calculated for all MLAER indices.

RESULTS

During propofol infusion, subjects were unresponsive, and MLAER components were significantly depressed when compared with the awake states (P < 0.001). The wavelet index Db3d4 was positive for awake and negative for unresponsive subjects with a prediction probability of 0.92.

CONCLUSION

These data show that automated wavelet analysis may be used to differentiate between awake and unresponsive states. The threshold value for the wavelet index allows easy recognition of awake versus unresponsive subjects. In addition, it is independent of subjective peak identification and offers the advantage of easy implementation into monitoring devices.

Effects of remifentanil/propofol in comparison with isoflurane on dynamic cerebrovascular autoregulation in humans

BACKGROUND

This study investigates the effects of remifentanil and propofol in comparison to isoflurane on dynamic cerebrovascular autoregulation in humans.

METHODS

In 16 awake patients dynamic cerebrovascular autoregulation was measured using transcranial Doppler sonography (TCD). Thereafter patients were intubated, ventilated with O2/air (FiO2=0.33) and randomly assigned to one of the following anesthetic protocols: group 1 (n=8): 0.5 microg x kg(-1) x min(-1) remifentanil combined with a propofol-target plasma concentration of 1.5 microg x ml(-1) group 2 (n=8): 1.8 % isoflurane (1.5 MAC). Following 20 min of equilibration the autoregulatory challenge was repeated. Arterial blood gases and body temperature were maintained constant over time.

STATISTICS

Mann-Whitney U-test and Wilcoxon signed-rank test.

RESULTS

Dynamic autoregulation was intact in all patients prior to induction of anesthesia expressed by an autoregulatory index (ARI) of 5.4+/-1.21 (mean+/-SD, group 1) and 5.9+/-0.98 (mean+/-SD, group 2). With remifentanil/propofol anesthesia dynamic autoregulation was similar to the awake state (group 1: ARI=4.9+/-0.88). In contrast, autoregulatory response was delayed with 1.5 MAC isoflurane (group 2, ARI=2.1+/-0.92) (P<0.05).

CONCLUSION

These data show that dynamic cerebrovascular autoregulation is maintained with remifentanil-based total intravenous anesthesia. This is consistent with the view that narcotics (and hypnotics) do not alter the physiologic cerebrovascular responses to changes in MAP. In contrast, 1.5 MAC isoflurane delays cerebrovascular autoregulation compared to the awake state.

Isoflurane slows inactivation kinetics of rat recombinant alpha1beta2gamma2L GABA(A) receptors: enhancement of GABAergic transmission despite an open-channel block

Recombinant alpha1beta2gamma2L gamma-aminobutyric acid (A) receptor (GABA(A)R) channels expressed in human embryonic kidney (HEK293) cells were used for patch-clamp experiments. The currents activated by brief pulses of GABA (10(-4) M) applied with a device for fast solution exchange to cells clamped in the whole-cell configuration mimicked GABA(A)R-mediated inhibitory postsynaptic currents. Isoflurane (ISO) at clinically relevant concentrations (0.6 mM) decreased the amplitude and prolonged the decay of the GABA-evoked response. To further detail the mechanism underlying the prolonged decay time, we made simulations based on these measurements. These simulations suggest that ISO slows the rate of GABA unbinding from the receptor. Under these conditions, ISO increases the GABA-induced charge transfer and, thus, could enhance GABAergic inhibition despite the concomitant open-channel block causing the decrease in the current amplitude.

Coadministered nitrous oxide enhances the effect of isoflurane on GABAergic transmission by an increase in open-channel block

Clinically relevant concentrations of isoflurane (ISO) and nitrous oxide (N2O) enhance chloride currents induced by activating gamma-aminobutyric acid(A) receptors (GABA(A)R). Channel blocking by ISO overcomes the enhancing effect at higher concentrations. In this study, the effect of coadministered ISO and N2O on responses evoked by GABA in transfected human embryonic kidney 293 cells carrying alpha1beta2gamma2L GABA(A)R was investigated. Patch-clamp recordings from these cells were performed in the whole cell mode. A piezo-driven "liquid filament" drug application system was used to apply solutions of GABA, ISO, and N2O. Increasing the concentration of ISO in steps from 0.15 to 1.2 mM resulted in a bell-shaped concentration-response curve for GABA-induced currents. The maximum increase in current (1.51 +/- 0.14-fold) was seen at 0.45 mM ISO (about 1 minimum alveolar concentration, EC50). N2O (29.2 mM) increased GABA-evoked currents 1.54 +/- 0.10-fold. The enhancing effects of ISO and N2O on the GABAergic response were not additive. However, a transient current, associated with the rapid withdrawal of ISO from the receptor, was markedly increased by N2O. Such rebound currents probably reflect the transition from a "channel-blocked" to a "reopened" state. An open-channel block at ligand-gated receptors can prolong postsynaptic currents. Thus, we conclude that coadministered N2O could increase the enhancing effect of ISO on the GABAergic transmission by an increase in open-channel block at the GABA(A)R.

Isoflurane blocks synaptic plasticity in the mouse hippocampus

BACKGROUND

The volatile anesthetic isoflurane depresses glutamatergic transmission. In this study, the authors investigated the effects of isoflurane on the induction of long-term potentiation (LTP) and long-term depression (LTD) in slices from the juvenile and adult mouse hippocampus. Both forms of synaptic plasticity involve the activation of glutamate receptors.

METHODS

Field excitatory postsynaptic potentials and excitatory postsynaptic currents from neurons in the CA1 area were evoked by stimulation of the Schaffer collateral-commissural pathway. Two independent synaptic inputs were stimulated. Clinically relevant concentrations (0.2-0.3 mM) of isoflurane were added to the perfusion solution.

RESULTS

Field excitatory postsynaptic potentials from slices of juvenile and adult mice were depressed to 37.3 +/- 6.1% and 58.3 +/- 7.4%, respectively, and excitatory postsynaptic currents were reduced to 36.7 +/- 5.4% by isoflurane. A brief tetanic stimulation (100 Hz, 1 s) induced stable LTP of field excitatory postsynaptic potentials. In the presence of isoflurane, tetanization failed to induce LTP. The effect of isoflurane on LTP induction was reversible and could be prevented by antagonizing gamma-aminobutyric acid type A receptors (GABAA). Low-frequency stimulation (1 Hz/900 pulses) induced LTD. In the presence of isoflurane, low-frequency stimulation failed to induce LTD.

CONCLUSIONS

The prevention of the isoflurane-induced depression of LTP by the GABAA antagonist picrotoxin suggests an involvement of GABAA receptors. An enhancement of the efficacy of GABA-mediated inhibitory synaptic transmission prevents the depolarization of the postsynaptic membrane during tetanus, necessary for the induction of use-dependent alteration of synaptic strength. An impairment of these processes may be a cause for the transient loss of recall and cognitive impairment after anesthesia in juvenile and adult brains.

Sevoflurane potentiates and blocks GABA-induced currents through recombinant alpha1beta2gamma2 GABAA receptors: implications for an enhanced GABAergic transmission

BACKGROUND AND OBJECTIVE

The gamma-aminobutyric acidA receptor (GABAAR) is a target for anaesthetic agents. We investigated the interactions of sevoflurane with a recombinant GABAAR. Emphasis was on the mechanism of block, as relevant open-channel block by a volatile anaesthetic would possibly explain prolonged GABAergic postsynaptic currents.

METHODS

The effect of sevoflurane on GABA-induced currents through recombinant alpha1beta2gamma2 GABAAR channels was studied (patch clamp; HEK293 cells). GABA 0.01 mM or 1 mM was applied alone or together with sevoflurane (0.05 mM to 5 mM).

RESULTS

Currents elicited by GABA 0.01 mM were increased by low sevoflurane concentrations to 183% and decreased by high sevoflurane concentrations (> 1 mM) to 34% (P < 0.05). Ten- to 90%-rise times of the currents were reduced by sevoflurane concentration dependently. At GABA (1 mM), peak currents and 10-90%-rise times decreased with increasing sevoflurane concentrations. A transient current increase was induced by discontinuation of GABA and sevoflurane. Such rebound currents indicate a reversal of an open-channel block by sevoflurane.

CONCLUSIONS

Sevoflurane (a) increases the apparent affinity of GABA to the GABAAR, as suggested by the decreased current rise times. This explains the enhancement of the currents induced by low GABA concentrations (0.01 mM). Additionally, sevoflurane (b) induces a picrotoxin-like open-channel block at the GABAAR. The reversal of the open-channel block elicits a delayed GABA response. These findings indicate at least two different sites of action of sevoflurane at this receptor that are both important for an enhanced GABAergic synaptic transmission.

Local GABAA Receptor Blockade Reverses Isoflurane’s Suppressive Effects on Thalamic Neurons In Vivo

Many in vitro effects of volatile anesthetics are known, but the mechanisms of action are still under debate. Because suppression of sensory perception is one of the major goals of general anesthesia, we studied the effects of isoflurane on the processing of somatosensory information in anesthetized rats. Local iontophoretic administration of the gamma-aminobutyric acid-A (GABA(A)) receptor antagonist bicuculline in the thalamic ventral posteromedial nucleus reversed suppressive effects of isoflurane on thalamocortical relay neurons (TCNs). The action potential discharges of TCNs (n = 23) in response to defined mechanical stimulation of receptive fields seen with small concentrations of isoflurane (0.79% +/- 0.01%, mean +/- SEM) were suppressed under large concentrations (1.44% +/- 0.04%). In addition, the tonic response pattern was lost, which initially encoded the information about the stimulus features. In 70% of TCNs, bicuculline administration reestablished the initially present tonic response pattern under large isoflurane concentrations. These results indicate that isoflurane suppresses somatosensory information transfer at the thalamic level in vivo, apparently by enhancing thalamic GABA(A) receptor-mediated inhibition.

Small-dose S(+)-ketamine reduces postoperative pain when applied with ropivacaine in epidural anesthesia for total knee arthroplasty

Reduction of nociceptive input through blockade of N-methyl-D-aspartate (NMDA) receptors has been reported. We compared the effects of epidural S(+)-ketamine versus placebo on postoperative pain in a randomized, double-blinded study in 37 patients undergoing unilateral knee arthroplasty. After lumbar epidural anesthesia with ropivacaine (10 mg/mL, 10-20 mL), 19 patients received 0.9% epidural saline, and 18 patients received 0.25 mg/kg epidural S(+)-ketamine 10 min before surgical incision. After surgery, patient-controlled epidural analgesia with ropivacaine was provided. During the first 8 h after surgery, visual analog scale pain rating was similar between groups. Twenty-four and 48 h after surgery, patients anesthetized with ropivacaine had higher visual analog scale ratings at rest and during movement (P < 0.05) than patients anesthetized with S(+)-ketamine and ropivacaine. Forty-eight hours after surgery, patients anesthetized with ropivacaine also consumed more ropivacaine (558 +/- 210 mg) (P < 0.01) than those anesthetized with S(+)-ketamine and ropivacaine (319 +/- 204 mg). Adverse events were similar between groups. Patients who received S(+)-ketamine and ropivacaine rated the quality of their pain therapy better than those who received ropivacaine alone (P < 0.05). We conclude that the combination of S(+)-ketamine and ropivacaine in epidural anesthesia increases postoperative pain relief when compared with ropivacaine.

IMPLICATIONS

Epidural S(+)-ketamine applied with ropivacaine before surgery is a rational approach to decrease injury-induced pain sensitization. Epidural blockade with an N-methyl-D-aspartate receptor antagonist and a local anesthetic may provide better analgesia in the postoperative period than a local anesthetic alone.