The Effect of Xenon on Spinal Dorsal Horn Neurons

Xenon reduces activation of transient receptor potential vanilloid type 1 (TRPV1) in rat dorsal root ganglion cells and in human TRPV1-expressing HEK293 cells

AIMS

Xenon provides effective analgesia in several pain states at sub-anaesthetic doses. Our aim was to examine whether xenon may mediate its analgesic effect, in part, through reducing the activity of transient receptor potential vanilloid type 1 (TRPV1), a receptor known to be involved in certain inflammatory pain conditions.

MAIN METHODS

We studied the effect of xenon on capsaicin-evoked cobalt uptake in rat cultured primary sensory neurons and in human TRPV1 (hTRPV1)-expressing human embryonic kidney 293 (HEK293) cells. We also examined xenon's effect on the phosphorylation of extracellular signal-regulated kinase 1/2 (ERK1/2) in the rat spinal dorsal horn evoked by hind-paw injection of capsaicin.

KEY FINDINGS

Xenon (75%) reduced the number of primary sensory neurons responding to the TRPV1 agonist, capsaicin (100 nM-1 μM) by ~25% to ~50%. Xenon reduced the number of heterologously-expressed hTRPV1 activated by 300 nM capsaicin by ~50%. Xenon (80%) reduced by ~40% the number of phosphorylated ERK1/2-expressing neurons in rat spinal dorsal horn resulting from hind-paw capsaicin injection.

SIGNIFICANCE

Xenon substantially reduces the activity of TRPV1 in response to noxious stimulation by the specific TRPV1 agonist, capsaicin, suggesting a possible role for xenon as an adjunct analgesic where hTRPV1 is an active contributor to the excitation of primary afferents which initiates the pain sensation.

Xenon inhibits excitatory but not inhibitory transmission in rat spinal cord dorsal horn neurons

BACKGROUND

The molecular targets for the promising gaseous anaesthetic xenon are still under investigation. Most studies identify N-methyl-D-aspartate (NMDA) receptors as the primary molecular target for xenon, but the role of alpha-amino-3-hydroxy-5-methyl-4-isoxazole-4-propionic acid (AMPA) receptors is less clear. In this study we evaluated the effect of xenon on excitatory and inhibitory synaptic transmission in the superficial dorsal horn of the spinal cord using in vitro patch-clamp recordings from rat spinal cord slices. We further evaluated the effects of xenon on innocuous and noxious stimuli using in vivo patch-clamp method.

RESULTS

In vitro, xenon decreased the amplitude and area under the curve of currents induced by exogenous NMDA and AMPA and inhibited dorsal root stimulation-evoked excitatory postsynaptic currents. Xenon decreased the amplitude, but not the frequency, of miniature excitatory postsynaptic currents. There was no discernible effect on miniature or evoked inhibitory postsynaptic currents or on the current induced by inhibitory neurotransmitters. In vivo, xenon inhibited responses to tactile and painful stimuli even in the presence of NMDA receptor antagonist.

CONCLUSIONS

Xenon inhibits glutamatergic excitatory transmission in the superficial dorsal horn via a postsynaptic mechanism. There is no substantial effect on inhibitory synaptic transmission at the concentration we used. The blunting of excitation in the dorsal horn lamina II neurons could underlie the analgesic effect of xenon.

Xenon blocks the induction of synaptic long-term potentiation in pain pathways in the rat spinal cord in vivo

BACKGROUND

Xenon's (Xe) mechanisms for producing anesthesia and analgesia are not fully understood. We tested the effect of Xe equilibrated in a lipid formulation or normal saline on spinal C-fiber-evoked potentials and on the induction of synaptic long-term potentiation (LTP).

METHODS

C-fiber-evoked field potentials were recorded in the superficial lumbar spinal cord in response to supramaximal electrical stimulation of the sciatic nerve. Anesthesia was maintained with isoflurane in one-third O2 and two-thirds N2O. Xe equilibrated at a concentration of 600 microL/mL of Lipofundin MCT(R) 20%, (n = 5) or solvent alone (n = 3), and Xe equilibrated at a concentration of 100 microL/mL of normal saline (n = 7) or saline alone (n = 7) was given IV under apnea. High-frequency stimulation of the sciatic nerve was applied 60 min after the injection of Xe-containing formulations or solvents [to induce LTP].

RESULTS

High-frequency stimulation potentiated C-fiber-evoked potentials to 156% +/- 14% (mean +/- sem) of control. Low-dose Xe in saline 0.9% blocked the induction of LTP. High-dose Xe equilibrated in MC(R) 20% showed no additional effect when compared with the solvent, which blocked the induction of LTP.

CONCLUSION

Low-dose Xe in saline 0.9% revealed no antinociceptive, but preventive, action in spinal pain pathways.

Corresponding minimum alveolar concentrations of isoflurane and isoflurane/nitrous oxide have divergent effects on thalamic nociceptive signalling

BACKGROUND

Suppression of nociceptive signalling in the thalamus is considered to contribute significantly to the anaesthetic state. Assuming additivity of anaesthetic mixtures, our study assessed the effects of corresponding minimum alveolar concentrations (MACs) of isoflurane and isoflurane/nitrous oxide on thalamic nociceptive signalling.

METHODS

Nociceptive response activity (elicited by controlled radiant heat stimuli applied to cutaneous receptive fields) of single thalamic neurons was compared in rats anaesthetized at approximately 1.1 and approximately 1.4 MAC isoflurane with that at approximately 1.1 and approximately 1.4 MAC isoflurane/nitrous oxide.

RESULTS

Under baseline anaesthesia ( approximately 0.9 MAC isoflurane), noxious stimulation elicited excitatory responses in all neurons (n = 19). These responses were uniformly suppressed at approximately 1.1 and approximately 1.4 MAC isoflurane. In contrast, at approximately 1.1 and approximately 1.4 MAC isoflurane/nitrous oxide, excitatory responses no different to baseline were still present in 64 and 37% of the neurons, respectively.

CONCLUSIONS

These data demonstrate a pronounced nitrous oxide-induced response variability. It appears that, with respect to thalamic transfer of nociceptive information, the interaction of isoflurane and nitrous oxide may not be compatible with the concept of additivity and that the antinociceptive potency of nitrous oxide is considerably less than previously reported.

Effect of xenon on catecholamine and hemodynamic responses to surgical noxious stimulation in humans

STUDY OBJECTIVE

To determine the effect of xenon in combination anesthesia with sevoflurane on the catecholamine and hemodynamic responses to surgical noxious stimulation in humans.

DESIGN

Randomized study.

SETTING

A university hospital.

PATIENTS

This study involved 32 female ASA physical status I and II patients, age 20-58 years, scheduled for abdominal hysterectomy.

INTERVENTIONS

Patients were randomly divided into 4 groups: group X50-S1.5, 50% xenon and 1.5% sevoflurane; group X70-S1.5, 70% xenon and 1.5% sevoflurane; group G70-S1.5, 70% nitrous oxide and 1.5% sevoflurane; and group S2.8, 2.8% sevoflurane. No premedication was administered to the patients, and anesthesia was induced by administration of sevoflurane in oxygen and 0.10 to 0.15 mg/kg of vecuronium. After tracheal intubation, the combination of anesthetics was started, and skin incision was performed after equilibration for more than 15 minutes.

MEASUREMENTS

Systolic blood pressure and heart rate (HR) were recorded, and the plasma concentrations of norepinephrine, epinephrine (E), and dopamine were measured 0, 2.5, 5, 7.5, 10, 12.5, and 15 minutes after skin incision.

MAIN RESULTS

The maximal increase in the E concentration and the values of the area under the curve for E were significantly smaller in the X50-S1.5 and X70-S1.5 groups compared with that in the S2.8 group (P<0.05). At 1 minute after incision, the HR in X50-S1.5 was significantly lower than those in G70-S1.5 and S2.8 groups and the HR in X70-S1.5 was lower than that in S2.8 group (P<0.01). The systolic blood pressure in S2.8 group at 1 minute was significantly higher than those of other groups (P<0.01).

CONCLUSION

Combination anesthesia using xenon and sevoflurane suppresses the plasma E concentration and hemodynamic response after skin incision more effectively than sevoflurane anesthesia alone.

The influence of xenon on regulation of the autonomic nervous system in patients at high risk of perioperative cardiac complications †

BACKGROUND

As xenon anaesthesia (XE) does not produce haemodynamic depression its use may be of benefit in patients at high risk of intraoperative haemodynamic instability and perioperative cardiac complications. XE (n=22) was compared with total i.v. anaesthesia (TIVA, n=22) for differences in autonomic regulation, peri- and postoperative performance.

METHODS

Patients undergoing abdominal aortic surgery were studied at five events: T1: baseline awake; T2: anaesthesia induction; T3: before aortic cross-clamping; T4: after aortic cross-clamping; T5: after aortic declamping. T3-T5: end-tidal xenon concentration 60 (5)%. Intraoperative analysis: heart rate, heart rate variability, blood pressure and cardiac output. Postoperative analysis: 24 h Holter ECG, intensive care unit and hospital stay, and patient's outcome after 6 months.

RESULTS

XE in contrast to TIVA increased parasympathetic and decreased sympathetic activity. Median low to high frequency decreased significantly in the XE group after start of XE (P<0.05) and remained significantly lower during all events after start of XE as compared with TIVA (P=0.0001). After start of XE heart rate of these patients was significantly lower as compared with TIVA (P=0.04). Cardiac output increased significantly in TIVA after aortic declamping (P<0.05). Outcome parameters did not differ significantly between groups.

CONCLUSIONS

XE patients demonstrated lower sympathetic and higher parasympathetic activity as compared with TIVA patients. This was reflected by significant differences in haemodynamics but did not correlate with a better postoperative outcome. Thus, it remains controversial whether XE provides benefits in high risk patients.

Inert gases as the future inhalational anaesthetics?

Of all the inert gases, only xenon has considerable anaesthetic properties under normobaric conditions. Its very low blood/gas partition coefficient makes induction of and emergence from anaesthesia more rapid compared with other inhalational anaesthetics. In experimental and clinical studies the safety and efficiency of xenon as an anaesthetic has been demonstrated. Xenon causes several physiological changes, which mediate protection of the brain or myocardium. The use of xenon might therefore be beneficial in certain clinical situations, as in patients at high risk for neurological or cardiac damage.

Xenon suppresses nociceptive reflex in newborn rat spinal cord in vitro; comparison with nitrous oxide

Although analgesic action of xenon has been reported, little is known about the effect of xenon at the spinal cord, which plays a crucial role in nociceptive transmission. We studied the effect of xenon on nociceptive reflex (the slow ventral root potential) and the monosynaptic reflex in neonatal rat spinal cord in vitro in comparison with nitrous oxide. Xenon (30%) and nitrous oxide (30%) were applied for 17 min through superfusing artificial cerebrospinal fluid. Xenon and nitrous oxide significantly reduced the amplitude of nociceptive reflex by approximately 70% and approximately 25%, respectively. Xenon and nitrous oxide also significantly reduced the amplitude of the monosynaptic reflex by approximately 35% and approximately 15%, respectively. These results indicate that xenon suppressed the synaptic transmission at the spinal cord, especially those of the slow ventral root potential, which reflect nociceptive transmission.

[Xenon anesthesia: from myth to reality]

OBJECTIVE

To analyze the current knowledge concerning xenon anaesthesia.

DATA SOURCES

References were obtained from computerized bibliographic research (Medline), recent review articles, the library of the service and personal files.

STUDY SELECTION

All categories of articles on this topic have been selected.

DATA EXTRACTION

Articles have been analysed for history, biophysics, pharmacology, toxicity and environmental effects and using prospect.

DATA SYNTHESIS

The noble gas xenon has anaesthetic properties that have been recognized 50 years ago. Xenon is receiving renewed interest because it has many characteristics of an ideal anaesthetic. In addition to its lack of effects on cardiovascular system, xenon has a low solubility enabling faster induction of and emergence from anaesthesia than with other inhalational agents. Nevertheless, at present, the cost and arety of xenon limit its widespread use in clinical practice. The developement of closed rebreathing system that allowed recycling of xenon and therefore reducing its waste has led to a recent interest in this gas. Reducing its cost will help xenon to find its place among anaesthetic agents. An European multicentric clinical trial under submission will contribute to the discussion of the opportunity for xenon introduction in anaesthesia.

Functional magnetic resonance imaging in rats subjected to intense electrical and noxious chemical stimulation of the forepaw

We examined whether cerebral activation to two different intense and painful stimuli could be detected using functional magnetic resonance imaging (fMRI) in alpha-chloralose anesthetized rats. Experiments were performed using a 9.4 T magnet and a surface coil centered over the forebrain. A set of gradient echo images were acquired and analyzed using our software based on fuzzy cluster analysis (EvIdent). Following the injection of 50 microl of formalin (5%) into the forepaw we observed a regional increase in signal intensity in the MR images in all animals. Anterior cingulate cortex, frontal cortex and sensory-motor cortex were some of the regions that activated frequently and often bilaterally. Surprisingly, activation appeared sequentially, often occurring first in either the right or the left hemisphere with a separation of seconds to minutes between peak activations. Morphine pre-treatment (1 mg/kg, i. v.) delayed and/or reduced the intensity of the activation resulting in a decrease in the overall response. Following episodes of intense electrical stimulation, produced by two brief stimulations (15 V, 0. 3 ms, 3 Hz) of the forepaw, activation was observed consistently in the sensory-motor cortex contralateral to the stimulation. Activation also occurred frequently in the anterior cingulate cortex, ipsilateral sensory-motor cortex and frontal cortical regions. All these regions of activation were markedly reduced during nitrous oxide inhalation. Treatment with morphine resulted in an inhibition of the activation response to electrical stimulation in most regions except for sensory-motor cortex. Thus, electrical and chemical noxious stimuli activated regions that are known to be involved in the central processing of pain and morphine modified the activation observed. fMRI combined with appropriate exploratory data analysis tools could provide an effective new tool with which to study novel analgesics and their effects on the CNS processing of pain in animal models.

Xenon suppresses the hypnotic arousal in response to surgical stimulation

STUDY OBJECTIVE

To evaluate the suppressive effects of xenon (Xe) on hypnotic arousal at skin incision.

DESIGN

Prospective, randomized study.

SETTING

Operating rooms at a university hospital.

PATIENTS

35 ASA physical status I and II patients presenting for elective lower abdominal surgery.

INTERVENTIONS

Patients were randomly assigned to receive one of the following regimens: 1.3 minimum alveolar concentration (MAC) isoflurane, 1.3 MAC sevoflurane, 0.7 MAC Xe with 0.6 MAC sevoflurane, 1 MAC Xe with 0.3 MAC sevoflurane, or 0.7 MAC nitrous oxide (N2O) with 0.6 MAC sevoflurane (n = 7 each group).

MEASUREMENTS AND MAIN RESULTS

The bispectral index (BIS) was measured at baseline, during anesthesia, and after skin incision. BIS increased significantly at skin incision from the values noted during anesthesia in the sevoflurane and N2O groups, whereas it remained stable at incision in the other three groups (mean change in BIS: 0 +/- 9 for isoflurane, 15 +/- 8 for sevoflurane, 5 +/- 6 for 0.7 MAC Xe, 4 +/- 11 for 1 MAC Xe, and 9 +/- 5 for N2O).

CONCLUSIONS

Unlike N2O, Xe was able to suppress hypnotic arousal in response to surgical stimulation when administered with sevoflurane.

Xenon has greater inhibitory effects on spinal dorsal horn neurons than nitrous oxide in spinal cord transected cats

Xenon (Xe) suppresses wide dynamic range neurons in cat spinal cord to a similar extent as nitrous oxide (N2O). The antinociceptive action of N2O involves the descending inhibitory system. To clarify whether the descending inhibitory system is also involved in the antinociceptive action of Xe, we compared the effects of Xe on the spinal cord dorsal horn neurons with those of N2O in spinal cord-transected cats anesthetized with alpha-chloralose and urethane. We investigated the change of wide dynamic range neuron responses to touch and pinch by both anesthetics. Seventy percent Xe significantly suppressed both touch- and pinch-evoked responses in all 12 neurons. In contrast, 70% N2O did not show significant suppression in touch- and pinch-evoked responses. These results suggest that the antinociceptive action of Xe might not be mediated by the descending inhibitory system, but instead may be produced by the direct effect on spinal dorsal horn neurons.

IMPLICATIONS

Xenon (Xe) is an inert gas with anesthetic properties. We examined the antinociceptive effects of Xe and nitrous oxide (N2O) in spinal cord-transected cats. Our studies indicate that Xe has a direct antinociceptive action on the spinal cord that is greater than that of N2O.