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From the *Department of Anesthesiology, Kansai Medical University, Osaka, Japan and
Magill Department of Anaesthesia, Chelsea and Westminster Hospital and Biophysics Section, The Blackett Laboratory, Imperial College of Science, Technology and Medicine, London, United Kingdom.
Address correspondence and reprint requests to Shinichi Nakao, MD, Department of Anesthesiology, Kansai Medical University, 10-15 Fumizono-cho, Moriguchi-shi, Osaka 570-8507, Japan. Address e-mail to nakaos{at}hirakata.kmu.ac.jp.
| Abstract |
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| Introduction |
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The mesolimbic dopamine system (MLDS) consists of dopaminergic neurons in the ventral tegmental area (VTA) and the regions to which these neurons project, notably the nucleus accumbens (NAC). It plays a role in the etiology of psychosis and represents important neurobiological components for the rewarding effects of abusing drugs (8). Noncompetitive NMDA receptor antagonists, such as phencyclidine, MK-801, and ketamine, which are substances of abuse, cause abnormal behaviors in animals, such as hyperlocomotion and head weaving (9,10). The NMDA antagonist-induced behavioral state has been preferred as an animal model of schizophrenia (11) and is associated with the activation of dopaminergic neurons in the VTA and dopamine release in the NAC (11,12). Blockade of the dopamine receptor (probably the D2 receptor) with haloperidol prevents this behavioral state (13) and the associated neuronal injury in the PC/RS (7). We (12) previously demonstrated that ketamine significantly increased the extracellular dopamine levels in the NAC, which was inhibited by pentobarbital. However, there are no data addressing the effects of nitrous oxide and xenon on dopamine release in the NAC. In the present study, we first investigated whether nitrous oxide and xenon increased the dopamine levels in the NAC, and then examined the effect on the ketamine-induced dopamine level increase in the NAC. Based on the contrasting effects of nitrous oxide and xenon on the PC/RS, we hypothesized that nitrous oxide, but not xenon, would increase the dopamine level in the NAC, and that nitrous oxide would enhance the ketamine-induced dopamine level increase in the NAC, but that xenon would inhibit it.
| METHODS |
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Preparation
The rats were placed within a stereotaxic frame under pentobarbital anesthesia (50 mg/kg i.p.). A stainless steel guide cannula was stereotaxically implanted unilaterally into the NAC with the following coordinates in relation to the bregma, AP: +1.6, ML: +1.2, DV: 6.2 mm, according to the atlas by Paxinos and Watson (14). The cannula was fixed to the skull with dental resin and stainless steel screws. The location of the probe within the NAC was verified by visual examination at the end of each experiment. The animals were allowed to recover for at least 2 days. On the day of study, the stylet was removed from the cannula and a microdialysis probe (A-I-12-2, Eicom, Kyoto, Japan) was inserted through the cannula. After placing the rat in a plexiglass box (25 x 25 x 25 cm) in which it could move freely, the microdialysis probe was connected to the perfusion pump and to the sample loop of an automated sample injector (Model 10; Eicom, Kyoto, Japan) with polyethylene tubing. The microdialysis probe was perfused continuously with an artificial cerebrospinal fluid solution (NaCl 147 mM, KCl 4 mM, CaCl2 2.3 mM) at a rate of 2.0 µL/min.
Experimental Protocol
Perfusate samples obtained during the first 120 min after implantation of the probe were discarded. The dialysate samples were then collected every 20 min. After verifying the stability of baseline dopamine levels, three consecutive basal samples were collected. Just after obtaining the basal samples (sample 0), gas exposure was initiated.
Experiment 1 (Effects of Nitrous Oxide and Xenon on Dopamine Levels in the NAC)
In Group 1 (n = 5; control group), the rats were exposed to 60% N2 and 40% O2 for 60 min. In Group 2 (n = 6), the rats were exposed to 60% nitrous oxide (0.27 MAC) (15) and 40% O2 for 60 min. In Group 3 (n = 6), the rats were exposed to 43% xenon (0.27 MAC) (16), 40% O2, and 17% N2 for 60 min.
Experiment 2 (Effects of Nitrous Oxide and Xenon on Ketamine-Induced Dopamine Levels in the NAC)
In Group 4 (n = 7), the rats were exposed to 60% N2 and 40% O2 for 70 min and 80 mg/kg ketamine was given i.p. 10 min after the initiation of gas exposure. In Group 5 (n = 5), the rats were exposed to 60% nitrous oxide (0.27 MAC) and 40% O2 for 70 min and 80 mg/kg ketamine was given i.p. 10 min after the initiation of gas exposure. In Group 6 (n = 6), the rats were exposed to 43% xenon (0.27 MAC), 40% O2, and 17% N2 for 70 min and 80 mg/kg ketamine was given i.p. 10 min after the initiation of gas exposure.
The xenon concentration was continuously monitored using a xenon gas monitor (Anzai Sogyo, Tokyo, Japan). The nitrous oxide concentration was continuously monitored using an anesthetic gas monitor, Type 1304® (Brüel and Kjær, Nærum, Denmark).
Measurement of Dopamine Levels
Dopamine levels in the dialysates were measured using a high-performance liquid chromatography column equipped with an electrochemical detector. The samples were injected by an autoinjector into an ODS-C18 reverse-phase column (2.1 x 150 mm CA-5ODS: Eicom, Kyoto, Japan) maintained at 25°C. The mobile phase consisted of 0.1 M phosphate buffer (pH 6.0) containing 50 mg/L EDTA2Na, 500 mg/L 1-octanesulfonate, and 20% methanol and was run at a flow rate of 230 µL/min. The oxidation potential of the graphite electrode was set at 450 mV against an Ag/AgCl reference electrode (ECD-300, Eicom, Kyoto, Japan). The detection limit of the assay was 0.2 pg/ 40 µL.
Statistics
Changes in extracellular dopamine levels over time are expressed as a percentage of each groups basal dopamine level (mean ± sd). After the dopamine level was considered to be stable, the basal level (sample 0) was taken as the mean of three initial collections (for 60 min) just before exposure to test drugs. Statistical evaluation was performed using the GraphPad Prism 4 (GraphPad Software, San Diego, CA). The differences within and among multiple groups over time were determined using two-way analysis of variance (ANOVA) with repeated measures, followed by Bonferronis test for multiple comparisons. P < 0.05 was considered statistically significant.
| RESULTS |
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Figure 1 shows the effects of nitrous oxide and xenon on dopamine levels in the NAC. Nitrous oxide (0.27 MAC) induced a progressive elevation of dialysate dopamine levels, reaching a statistical significance after 40 min of nitrous oxide exposure; a maximum of 144% of the basal level was reached in the third perfusate sample (6080 min epoch of nitrous oxide exposure). After the exposure to nitrous oxide was discontinued, the dopamine level gradually decreased to the basal level. The dopamine levels in the second and third perfusate samples of the nitrous oxide group were significantly higher than those in the control and xenon groups. In the fourth perfusate sample, the dopamine level in the nitrous oxide group was also significantly higher than in the xenon group. Conversely, xenon (0.27 MAC) did not affect the dopamine levels in the NAC, although a statistically insignificant decrease was seen.
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Figure 2 shows the effect of nitrous oxide or xenon on ketamine-induced dopamine levels in the NAC. Ketamine, at a dose of 80 mg/kg i.p., induced a progressive elevation of dopamine levels in the NAC, reaching statistical significance in the second perfusate sample (4060 min after ketamine i.p.). The dopamine levels remained elevated until the fourth sample (80100 min), reaching a maximum of 243% of the basal level in the third sample. Nitrous oxide (0.27 MAC) did not affect the ketamine-induced dopamine levels in the NAC. On the other hand, the dopamine level in the third sample (6080 min) of ketamine with 40% O2 was significantly higher than that of ketamine with xenon (0.27 MAC). Also, there was no significant increase in dopamine levels by ketamine with xenon.
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| DISCUSSION |
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Several potential limitations of our experiment should be considered. First, we did not systematically examine the behavioral changes in response to the test drugs because the main purpose of this experiment was to investigate dopamine release in the NAC. A correlation between dopamine release in the NAC and abnormal behavior induced by NMDA receptor antagonists such as MK-801 has been reported (11). Second, using the present experimental protocol, it was impossible to clarify whether the increase in dopamine levels in the NAC was due to an increase in dopamine release or a decrease in re-uptake. Because NMDA receptor antagonists activate VTA dopamine neurons (17), it is more probable that the dopamine level increase in the NAC induced by both ketamine and nitrous oxide can be attributed to an increase in dopamine release, rather than a decrease in dopamine re-uptake.
Nitrous oxide and xenon display a similar spectrum of receptor activity: both nitrous oxide and xenon potently inhibit not only NMDA receptors (3,4,18), but also nicotinic acetylcholine (nAch) receptors comprising the
4ß2 subunits, with no or little effect on
-aminobutyric acidA receptors and non-NMDA glutamate receptors (4,18). These gaseous anesthetics also exhibit similar clinical features; for example, both have analgesic action and cause similar electroencephalographic changes, exhibiting attenuation of
wave at lower concentrations, with the appearance of
- and
-wave activity at higher concentrations (19). A bispectral index value is not a reliable indicator for hypnosis in either xenon or nitrous anesthesia (20,21). However, their properties are quite different in various respects.
In the present study, we demonstrated that xenon can be differentiated from nitrous oxide with respect to its effects on the MLDS, suggesting that the contrasting neuropsychological properties of xenon and nitrous oxide can be, at least partially, attributed to their different effects on the MLDS. Both xenon and nitrous oxide are not only noncompetitive NMDA receptor antagonists, but also nAch receptor inhibitors (18). Although dopamine transmission in the MLDS is also regulated by cholinergic input (22), the effects of nitrous oxide and xenon on the MLDS do not seem to be mediated predominantly by their inhibitory effects on nAch receptors. This is because the dopamine release would have been inhibited if they acted predominantly at nAch receptors. Also, it is well established that NMDA receptor antagonists activate the MLDS and, consequently, induce psychotomimetic effects. Conversely, the inhibition of ketamine-induced dopamine release by xenon may be partly attributed to nAch receptor inhibition by xenon.
Xenon might not only have less psychotomimetic and addictive effects than nitrous oxide, but may also be protective against NMDA receptor antagonist-induced psychotomimetic effects and neurotoxicity, because dopamine is neurotoxic in some situations (23). Indeed, it has been demonstrated that xenon has neuroprotective effects in both in vivo and in vitro studies (24). Interestingly, the contrasting effects of nitrous oxide and xenon on the MLDS are consistent with their effects on the PC/RS, and independent studies have suggested that the MLDS and the PC/RS are the brain regions responsible for the psychotomimetic effects of NMDA receptor antagonists and the etiology of schizophrenia. Aalto et al. (25) recently demonstrated that ketamine increased dopamine release in the PC/RS not only in rats, but also in humans, although the role of the dopamine system in the PC/RS has not been clarified. Further studies will be required to elucidate the precise role of the dopamine system in the PC/RS and the relationship and/or neuronal networks between the PC/RS and the MLDS.
In conclusion, we demonstrated that nitrous oxide, but not xenon, increased the dopamine level in the NAC in freely moving rats, and that xenon inhibited the ketamine-induced increase in the dopamine level.
| Footnotes |
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Supported by Japan Society for the Promotion of Science Grant 16591572.
Nick Franks and Mervyn Maze are co-founders of an Imperial College London spin-out company, Protexeon; Protexeon had been issued with patents for the use of xenon as a neuroprotectant. Protexeon was acquired by Air Products. However, no funding was received from Air Products for this study.
| REFERENCES |
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