Rapidly developing tolerance to acute exposures to anesthetic agents

Nitrous oxide analgesia in humans: acute and chronic tolerance

Electrical tooth stimulation was used to investigate whether humans develop tolerance to nitrous oxide (N(2)O) analgesia within a single administration as well as over repeated administrations. In a double-blind cross-over experiment, 77 subjects received a 40-min administration of 38% N(2)O at one session and placebo gas at the other. The sessions were separated by 1 week and the order of gas administration was counterbalanced. Acute analgesic tolerance developed for pain threshold but not for detection threshold. There was no evidence of a hyperalgesic rebound effect following cessation of the N(2)O administration. In a second double-blind experiment, 64 subjects received both 30-min of placebo gas and 30-min of 35% N(2)O, separated by a 35-min gas wash-out period, during each of five sessions. Sensory thresholds were assessed prior to drug or placebo administration (baseline) and between 7-12 and 25-30 min of gas administration. A control group of 16 subjects received only placebo gas at these five sessions. During a sixth session, the experimental procedures were similar to the previous sessions except that the control group received N(2)O for the first time and the experimental group was sub-divided to test for conditioned drug effects. For both detection and pain threshold measures, acute tolerance developed during the initial N(2)O exposure and chronic tolerance developed over repeated administrations. Although chronic tolerance developed, a test for Pavlovian drug conditioning found no evidence of conditioned effects on sensory thresholds. In conclusion, acute and chronic tolerance develop to N(2)O's analgesic effects in humans.

Neurobiology of nitrous oxide-induced antinociceptive effects

Nitrous oxide (N2O), or laughing gas, has been used for clinical anesthesia for more than a century and is still commonly used. While the anesthetic/hypnotic mechanisms of N2O remain largely unknown, the underlying mechanisms of its analgesic/antinociceptive effects have been elucidated during the last several decades. Evidence to date indicate that N2O induces opioid peptide release in the periaqueductal gray area of the midbrain leading to the activation of the descending inhibitory pathways, which results in modulation of the pain/nociceptive processing in the spinal cord. The types of opioid peptide induced by N2O and the subtypes of opioid receptors that mediate the antinociceptive effects of N2O appear to depend on various factors including the species and/or strain, the regions of the brain, and the paradigms of behavior testing used for the experiments. Among three types of descending inhibitory pathways, the descending noradrenergic inhibitory pathway seems to play the most prominent role. The specific elements involved are now being resolved.

Evaluation and comparison of the pharmacokinetic and pharmacodynamic properties of allopregnanolone and pregnanolone at induction of anaesthesia in the male rat

We have evaluated and compared the pharmacokinetic and pharmacodynamic properties of allopregnanolone and pregnanolone at induction of anaesthesia in male rats. A threshold method was used, and the first burst suppression period of 1 s or more in the EEG was selected as the end-point after fairly slow infusions. An optimal dose of 4.0 mg kg(-1) min(-1) was noted for both steroids. Brain concentrations were low at low infusion rates, indicating that acute tolerance was not occurring. Significant positive correlations were noted between dose rate and serum concentrations of allopregnanolone (r = 0.94, P<0.001) and pregnanolone (r = 0.88, P<0.001). Such correlations were also seen in striatum, cerebellum, cortex and muscle for both steroids (P<0.01). Despite changing infusion rates, the concentrations of both steroids in brainstem, hippocampus and fat remained stable. Because no correlation between infusion rate and steroid concentration was noted in the brainstem and hippocampus, these two brain areas may be regarded as primary sites of action for allopregnanolone and pregnanolone. Pregnanolone concentrations in the brainstem and hippocampus were significantly higher than those of allopregnanolone, suggesting that allopregnanolone was more potent than pregnanolone in inducing anaesthesia.

Nitrous oxide-induced hypothermia in the rat: acute and chronic tolerance

Although inhalation of nitrous oxide (N2O) causes hypothermia in rats, there is a paucity of information as to whether tolerance develops to this effect. The purpose of this study was to determine whether tolerance to N2O hypothermia develops within a single administration as well as over repeated administrations. Temperature was measured telemetrically by implanting intraperitoneal thermal sensors/transmitters in male Long-Evans rats. Experimental rats received an initial 2-h exposure to 60% N2O and became hypothermic relative to controls breathing placebo gas. Only a few rats demonstrated evidence of acute tolerance over the 120 min. Over the next 10 days, the experimental rats received five additional 30-min exposures to 60% N2O and five 30-min exposures to placebo while the control rats received only placebo gas exposures. Chronic tolerance developed to N2O hypothermia over these repeated administrations. A test for Pavlovian drug conditioning found no evidence that conditioned temperature effects contributed to chronic tolerance development. In a second experiment, naive rats were given a 380-min exposure to 60% N2O and a 380-min exposure to placebo gas in a counterbalanced order. Acute tolerance did develop to N2O hypothermia, with the recovery of temperature beginning after a mean of 141 min of gas administration. Hence, both acute and chronic tolerance develop to N2O's hypothermic effects in rats.

Nonlinear additivity of nitrous oxide and isoflurane potencies in rats

PURPOSE

To test the hypothesis that the MAC values of nitrous oxide (N2O) and isoflurane were not linearly additive, as theorized by the postulated mode of action based on lipid solubility, in a rat model.

METHODS

Eight Long Evans rats were randomly assigned to order of measurement of MAC for isoflurane and N2O alone and in combination using standard 45 sec supramaximal electrical stimulation (50 volts x 10 msec duration pulses at 50.sec-1 applied for 45 sec s.c. to the lower abdominal groin area). The MAC of N2O was measured at hyperbaric compression to 2.25 atmospheres absolute, 1710 mmHg.

RESULTS

The MAC values found were: isoflurane -0.98 +/- 0.12 and N2O - 159 +/- 12 volume (vol)%, or 1.59 +/- 0.12 atmospheres absolute (ATA) (All values are mean +/- standard deviation). The linear additivity theory suggests % MAC agent A + % MAC agent B = 1.0. However, % MAC isoflurane + % MAC N2O = 1.37 +/- 0.15 (P < .001).

CONCLUSION

Nonlinear additivity was demonstrated with direct MAC measurement for both isoflurane and N2O in rats. This suggests an agonist-antagonist relationship.

Effects of isoflurane on in vivo release of acetylcholine in the rat cerebral cortex and striatum

BACKGROUND

Acetylcholine (ACh) is one of the major excitatory neurotransmitters in the central nervous system, and changes in neural activity induced by anesthesia alter the release of ACh. However, the effects of isoflurane, one of the most widely used volatile anesthetics, on ACh release are not known. The present study attempts to clarify the dose-effect relationship of isoflurane on the in vivo release of ACh in rat brains.

METHODS

Changes in the extracellular concentration of ACh and choline in the cerebral cortex and striatum induced by 0.5, 1.0, and 1.5 minimum alveolar concentration (MAC) of isoflurane were determined using a brain microdialysis technique.

RESULTS

In the cortex, the ACh release decreased to 30.8+/-10.1 (mean+/-SEM), 10.2+/-4.1, and 8.1+/-2.9% of basal value by increasing doses of isoflurane, and in the striatum, to 73.3+/-4.4, 49.2+/-4.2, and 40.7+/-4.5%. The ACh release rapidly recovered control levels with the discontinuance of isoflurane. Choline concentration was not changed significantly by isoflurane except for a decrease to 74.8+/-4.6% in the striatum by 0.5 MAC. In both the cortex and striatum, the choline concentration decreased with the discontinuance of isoflurane to 70.3+/-13.3, and 68.2+/-5.4%, respectively.

CONCLUSION

The fact that all classic anesthetics reported previously, as well as isoflurane, reduce ACh release supports the hypothesis that the suppression of cholinergic cells is, at least in part one of the mechanisms of anesthesia.

Nitrous oxide depresses somatocardiac sympathetic A- and C-reflexes in anesthetized rats

Effect of nitrous oxide (N2O) on the somatosympathetic A- and C-reflexes was investigated using artificially ventilated rats anesthetized with alpha-chloralose and urethane. Somatocardiac sympathetic A- and C-reflexes were elicited in the inferior cardiac nerve by electrical stimulation of A and C afferent fibers of the tibial nerve, respectively. Both reflexes were depressed by inhalation of N2O for 20 min. The depression was greater in the C-reflex than in the A-reflex. The depressive effects of N2O on both reflexes were unchanged after pretreatment with intravenous naloxone (0.2 or 2.0 mg/kg) or by prolongation of the inhalation of N2O for 2 h. These results suggest that the opioid receptor is not involved and that acute tolerance is not developed in the depressive action of N2O on the somatosympathetic A- and C-reflexes.

Age-dependent development of acute tolerance to propofol and its distribution in a pharmacokinetic compartment-independent rat model

BACKGROUND

Previous animal and clinical studies have shown a dose-dependent increase in propofol concentrations in blood at wakening after propofol infusion. A development of an acute tolerance to propofol has been proposed. To elucidate this, brain concentrations of propofol at the time of an EEG end point was used as a measure of CNS sensitivity.

METHODS

Twenty young or 20 old rats were assigned to either induction of anaesthesia with propofol or induction followed by maintenance of anaesthesia with intermittent propofol infusions during 60 minutes. All rats were killed at the EEG end point and samples of cerebral cortex, hippocampus, striatum, brain stem, cerebellum, blood, muscle and fat tissue were submitted to HPLC analysis of propofol concentrations. A new model describing the infusion requirements is introduced.

RESULTS

The estimated infusion rate during maintenance declined exponentially with time at a slower rate in young compared with old rats. Old rats had higher propofol concentrations in blood, hippocampus, striatum, brain stem, cerebellum, muscle and fat after 60 minutes compared with induction. In contrast, young animals had no significant change in brain concentrations, but the propofol concentrations in muscle and fat were increased.

CONCLUSION

It is concluded that maintenance of propofol anaesthesia may alter the CNS sensitivity to propofol in old animals, i.e. an acute tolerance may develop. Young animals do not seem to be capable of developing an acute tolerance to propofol. The results also support previous findings that a redistribution is important for the rapid clearance of propofol from blood.

Lack of acute tolerance development to the subjective, cognitive, and psychomotor effects of nitrous oxide in healthy volunteers

A crossover, double-blind trial was conducted using eleven healthy volunteers to determine whether and the degree to which acute drug tolerance occurred to the subjective, cognitive, and psychomotor effects of a range of subanesthetic nitrous oxide doses (0, 10, 20, 30, and 40%). There was little evidence of acute drug tolerance to the subjective measures or to the cognitive/psychomotor impairing effects of nitrous oxide at any of the concentrations tested over the course of the 120-min inhalation.

Acute tolerance to nitrous oxide in humans

The goal of this research was to determine whether the level of analgesia produced by nitrous oxide remains constant for the duration of a typical dental procedure or whether acute tolerance reduces the drug's efficacy. A computer-controlled stimulator delivered brief (approx. 1 msec) electrical pulses to a vital maxillary incisor which had been found to have normal sensitivity in a preliminary session. Subjects were trained to indicate the occurrence of a barely perceptible sensation (i.e., detection threshold) as well as a minimally painful sensation (i.e., pain threshold). On the experimental day, all subjects breathed a non-odorized placebo gas mixture during a 10-min baseline condition, and were then randomly assigned to receive either an odorized placebo gas mixture or an odorized 35-40% nitrous oxide/oxygen gas mixture for 46 min. Detection and pain thresholds were assessed repeatedly during the baseline and gas exposure conditions. Placebo control subjects had little change of either sensory threshold. Subjects breathing nitrous oxide significantly increased both detection and pain thresholds within 2-8 min following the onset of the drug. However, maintenance of the drug's effect was not consistent between subjects, despite continuous administration of a constant concentration of nitrous oxide. Some subjects had a relatively constant elevation of sensory thresholds throughout the nitrous oxide administration period, and others returned to baseline sensitivity values and therefore were acutely tolerant.

Cyclic alteration in the anticonvulsant effect of nitrous oxide in rats

The anticonvulsant action of nitrous oxide and its time course were studied in rats. Bicuculline, a GABA-receptor antagonist, was administered intravenously at a rate of 0.2 mg.kg(-1).min(-1) during exposure to air (n = 60) or 75% nitrous oxide in oxygen (n = 80). The convulsant dose of bicuculline was determined. The rats were divided into subgroups according to the duration of exposure to air or nitrous oxide, from 0 to 120 min at 15 min intervals. Although the convulsant dose of bicuculline was consistent in the air group (1.03 +/- 0.06 mg.kg(-1), mean +/- SEM), it showed two peaks at 30- and 90 min exposures to nitrous oxide. The threshold dose in the nitrous oxide group was significantly higher than in the air group at only 15- and 30 min exposures (1.50 +/- 0.16, 2.15 +/- 0.25 mg.kg(-1), respectively, P < 0.05). We conclude that nitrous oxide has an anticonvulsant action against bicuculline-induced seizure, and that a cyclic nature exists in its action.

Tolerance to nitrous oxide in volunteers

Nociception and loss of awareness during exposure to anaesthetic concentration of nitrous oxide (N2O) were studied in eight male medical students. The cold water nociception test, where a hand is immersed in 0 degree C stirred water, was used for measurement of nociception. At irregular intervals an auditory command was given to oppose two fingers, and this served to monitor consciousness. The selected inspiratory concentration of N2O used per individual was sufficient to induce a loss of consciousness for more than 2.5 min, within 10 min of exposure to N2O. This concentration of N2O varied from 60% to 80%. The experimental exposure to N2O lasted 3 h. In all volunteers significant antinociception was observed within 2 min of exposure to N2O. The maximal analgesic effect was observed between 20 and 30 min of exposure to N2O. The analgesic effect of N2O gradually decreased and was absent in all eight volunteers within 150 min. Two volunteers regained consciousness at 77 and 91 min of exposure, whilst still breathing 60 and 80% N2O. These results show that tolerance to antinociceptive effects of N2O in man rapidly develops and that awareness may occur in some volunteers during prolonged exposure to N2O.

Enkephalinase inhibition prevented tolerance to nitrous oxide analgesia in rats

Tolerance to nitrous oxide (N2O) antinociception was studied in rats in accordance with the Randall-Selitto pressure nociception test. Both N2O (70% in 30% O2) and the relatively selective enkephalinase inhibitor phosphoramidon (350 micrograms i.c.v.), which blocks the biotransformation of enkephalins, were administered. They both induced a significant analgesic effect which vanished within 45 min. The rapidly developed tolerance to N2O analgesia does not affect the anaesthetic state since the animals remained motionless for the duration of exposure lasting 3 h. In the animals treated with the enkephalinase inhibitor phosphoramidon, no development of tolerance to N2O-antinociception occurred during the exposure lasting 3 h. The results indicate that tolerance to N2O analgesia can be abolished by activation of the enkephalinergic system, which might suggest a possible insufficiency of this system during tolerance to N2O.

Lack of tolerance to cyclopropane and cross-tolerance to halothane in rats

Cyclopropane requirement (MAC) in rats was determined before, during, and after repeated exposure to anaesthetic concentrations of cyclopropane and halothane for one hour every day for 20 and 30 days, respectively. MAC values during and after exposure ranged from 93 to 103 per cent (mean = 99 per cent) of control values, which indicates a lack of significant tolerance to cyclopropane or a cross-tolerance between cyclopropane and halothane.

Murine auto- and cross-tolerance to volatile anaesthetics

Auto-tolerance and cross-tolerance to halothane, isoflurane and enflurane were tested on 36 mice divided into three equal groups. Each group was first exposed to increasing concentrations of either of the three anesthetics on 13 occasions. The concentration at which each mouse lost its righting reflex during successive exposures in a rotating cage was noted. Cross-tolerance was assessed by comparing the number of mice which had lost their righting reflexes during their first exposure to a given anaesthetic agent to the number which lost it after having been exposed to another anaesthetic. All animals developed auto-tolerance to halothane, isoflurane and enflurane. Cross-tolerance was noted only between mice exposed to isoflurane and enflurane and between mice exposed to halothane and subsequently anaesthetized with isoflurane, but not vice versa.

Physical dependence on nitrous oxide in mice: resemblance to alcohol but not to opiate withdrawal

Mice of two strains, Crl:CD-1(1CR)Br and C57BL6, were exposed to nitrous oxide at concentrations of 50, 65 and 80 per cent for 34 or 68 hours. Cessation of nitrous oxide resulted in characteristic convulsions similar to those seen in alcohol withdrawal in all mice. These peaked in severity within 2-3 minutes after removal from nitrous oxide and declined over 6 hours. The severity and duration of these convulsions were related to the nitrous oxide concentration and duration of exposure. Naloxone or naltrexone produced no significant increase in severity of convulsions. The narcotic antagonists did not precipitate acute weight loss or characteristic jumping behaviour seen in animals dependent on opiates. These results demonstrate that chronic exposure to nitrous oxide results in development of physical dependence which resembles alcohol and not opiate dependence. Analgesia and physical dependence produced by nitrous oxide appear to be mediated through separate mechanisms.