Rat strain minimally influences anesthetic and convulsant requirements of inhaled compounds in rats

Comparison of the respiratory effects of commonly utilized general anaesthesia regimes in male Sprague-Dawley rats

Background: Respiratory parameters in experimental animals are often characterised under general anaesthesia. However, anaesthesia regimes may alter the functional and mechanical properties of the respiratory system. While most anaesthesia regimes have been shown to affect the respiratory system, the effects of general anaesthesia protocols commonly used in animal models on lung function have not been systematically compared. Methods: The present study comprised 40 male Sprague-Dawley rats divided into five groups (N = 8 in each) according to anaesthesia regime applied: intravenous (iv) Na-pentobarbital, intraperitoneal (ip) ketamine-xylazine, iv propofol-fentanyl, inhaled sevoflurane, and ip urethane. All drugs were administered at commonly used doses. End-expiratory lung volume (EELV), airway resistance (Raw) and tissue mechanics were measured in addition to arterial blood gas parameters during mechanical ventilation while maintaining positive end-expiratory pressure (PEEP) values of 0, 3, and 6 cm H2O. Respiratory mechanics were also measured during iv methacholine (MCh) challenges to assess bronchial responsiveness. Results: While PEEP influenced baseline respiratory mechanics, EELV and blood gas parameters (p < 0.001), no between-group differences were observed (p > 0.10). Conversely, significantly lower doses of MCh were required to achieve the same elevation in Raw under ketamine-xylazine anaesthesia compared to the other groups. Conclusion: In the most frequent rodent model of respiratory disorders, no differences in baseline respiratory mechanics or function were observed between commonly used anaesthesia regimes. Bronchial hyperresponsiveness in response to ketamine-xylazine anaesthesia should be considered when designing experiments using this regime. The findings of the present study indicate commonly used anaesthetic regimes allow fair comparison of respiratory mechanics in experimental animals undergoing any of the examined anaesthesia protocols.

Ethanol reduces the minimum alveolar concentration of sevoflurane in rats

A high number of trauma patients are under the influence of alcohol. Since many of them need immediate surgical procedures, it is imperative to be aware of the interaction of alcohol with general anesthesia. To counter challenges that arise from clinical studies, we designed an animal experiment in which 48 adult Wistar rats either received 1 g · kg-1 ethanol, 2 g · kg-1 ethanol or placebo via intraperitoneal application. Subsequently, they were anesthetized with an individual concentration of sevoflurane. The minimum alveolar concentration (MAC) of the different groups was assessed using Dixon's up-and-down design and isotonic regression methods. The bootstrap estimate of the MAC of sevoflurane in the placebo group was 2.24 vol% (95% CI 1.97-2.94 vol%). In the low dose ethanol group, the bootstrap estimate was 1.65 vol% (95% CI 1.40-1.98 vol%), and in the high dose ethanol group, it was 1.08 vol% (95% CI 0.73-1.42 vol%). We therefore report that intraperitoneal application of 1 g · kg-1 or 2 g · kg-1 ethanol both resulted in a significant reduction of the MAC of sevoflurane in adult Wistar rats: by 26.3% and 51.8% respectively as compared to placebo.

Aversion to Desflurane and Isoflurane in Sprague-Dawley Rats (Rattus Norvegicus)

Simple Summary

Euthanasia is one of the most commonly performed procedures in laboratory rodents, as the majority of animals are killed upon project completion or when humane endpoints have been reached. Overdose with carbon dioxide gas remains a widely used killing method, despite evidence it is aversive to rodents. The inhalant anesthetic isoflurane is a refinement to overdose with carbon dioxide, but also elicits aversion in rodents. The inhalant anesthetic desflurane has a faster onset of action than isoflurane and may therefore offer further refinement. In this study, rat aversion to desflurane and isoflurane was compared. Isoflurane and desflurane were similarly aversive; however, desflurane exposure resulted in a shorter time to achieve recumbency, shortening any period of potential distress. Therefore, desflurane represents a refinement over the use of isoflurane.

Abstract

Carbon dioxide and isoflurane are widely used for killing rats, yet may not truly achieve “euthanasia”, because they elicit aversion. The inhalant anesthetic desflurane is faster acting than isoflurane, representing a potential refinement. Using an aversion-avoidance paradigm, 24 rats were exposed to isoflurane or desflurane (n = 12 per group) at initial exposure. Fourteen rats were then re-exposed to isoflurane or desflurane (n = 7 per group), after a 7 days washout period. Initial exposure: time to recumbency was faster for desflurane than isoflurane (p = 0.0008, 95% CI [-12.9 to 32.6 s]), with 9/12 and 6/12 rats becoming recumbent, respectively. At initial exposure, there was no difference between groups in time to withdrawal (p = 0.714). At re-exposure, all rats withdrew and no rats became recumbent. Time to withdrawal at re-exposure did not differ between treatment groups (p = 0.083). Compared to initial exposure, time to withdrawal during re-exposure was similar for isoflurane (p = 0.228) and faster with desflurane (p = 0.012, 95% CI [19.1 to 49.5 s]). Isoflurane and desflurane are similarly aversive, with aversion increasing at re-exposure. The shorter time from exposure to recumbency with desflurane indicates that any distress is of a shorter duration when compared with isoflurane.

Desflurane impairs hippocampal learning on day 1 of exposure: a prospective laboratory study in rats

Background

Quick and complete recovery of cognitive function after general anesthesia is desirable, particularly for working-age patients. Desflurane is less likely to have long-term effects than older-generation inhalational anesthetics, however, its short-term effects have not been fully investigated. Our objective was to elucidate the short-term effects of desflurane exposure on learning and memory in young adult rats.

Methods

Seven-week old male Sprague–Dawley rats were exposed to air (control), or desflurane at 0.7 or 1.2 minimum alveolar concentration (MAC) for 2 h (day 0). The inhibitory avoidance (IA) test was performed on day 1 to delineate the effects on contextual learning. Separate groups of control and 1.2 MAC desflurane animals underwent the IA test on days 3 and 7 to examine the time-dependent changes. Because the IA test is known to be dependent on the long-term potentiation (LTP) of the hippocampus and the trafficking of the GluR1 subunit of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor into the synapses, the effects of 1.2 MAC desflurane on these phenomena were evaluated on day 1.

Results

Desflurane at 1.2 MAC, but not 0.7 MAC, significantly decreased the IA latencies on day 1 compared with the control (one-way ANOVA, F [2,48] = 5.974, P = 0.005, post hoc Tukey’s, mean difference [95% confidence interval], control vs. 1.2 MAC, 168 [49.9 to 287], P = 0.004; control vs. 0.7 MAC, 67.5 [− 51.2 to 186], P = 0.362). The latencies were not affected on days 3 and 7 (day 3, control vs. desflurane, P = 0.861; day 7, control vs. desflurane, P > 0.999). Consistently, hippocampal LTP on day 1 was significantly suppressed in the desflurane group compared with the control group (P = 0.006). Moreover, immunoblotting analysis of synaptic GluR1 expression revealed that desflurane exposure significantly suppressed GluR1 delivery to the synapses after IA training.

Conclusion

Exposure to a relatively high concentration of desflurane caused reversible learning and memory impairment in young adult rats associated with suppression of GluR1 delivery to the synapses in the hippocampus.

Genetic variability affects absolute and relative potencies and kinetics of the anesthetics isoflurane and sevoflurane in Drosophila melanogaster

Genetic variability affects the response to numerous xenobiotics but its role in the clinically-observed irregular responses to general anesthetics remains uncertain. To investigate the pharmacogenetics of volatile general anesthetics (VGAs), we developed a Serial Anesthesia Array apparatus to expose multiple Drosophila melanogaster samples to VGAs and behavioral assays to determine pharmacokinetic and pharmacodynamic properties of VGAs. We studied the VGAs isoflurane and sevoflurane in four wild type strains from the Drosophila Genetic Reference Panel, two commonly used laboratory strains (Canton S and w 1118 ), and a mutant in Complex I of the mitochondrial electron transport chain (ND23 60114 ). In all seven strains, isoflurane was more potent than sevoflurane, as predicted by their relative lipid solubilities, and emergence from isoflurane was slower than from sevoflurane, reproducing cardinal pharmacokinetic and pharmacodynamic properties in mammals. In addition, ND23 60114 flies were more sensitive to both agents, as observed in worms, mice, and humans carrying Complex I mutations. Moreover, we found substantial variability among the fly strains both in absolute and in relative pharmacokinetic and pharmacodynamic profiles of isoflurane and sevoflurane. These data indicate that naturally occurring genetic variations measurably influence cardinal pharmacologic properties of VGAs and that flies can be used to identify relevant genetic variations.

Slowing of the hippocampal θ rhythm correlates with anesthetic-induced amnesia

BACKGROUND

Temporary, antegrade amnesia is one of the core desirable endpoints of general anesthesia. Multiple lines of evidence support a role for the hippocampal θ rhythm, a synchronized rhythmic oscillation of field potentials at 4-12 Hz, in memory formation. Previous studies have revealed a disruption of the θ rhythm at surgical levels of anesthesia. We hypothesized that θ-rhythm modulation would also occur at subhypnotic but amnestic concentrations. Therefore, we examined the effect of three inhaled agents on properties of the θ rhythm considered critical for the formation of hippocampus-dependent memories.

METHODS

We studied the effects of halothane and nitrous oxide, two agents known to modulate different molecular targets (GABAergic [γ-aminobutyric acid] vs. non-GABAergic, respectively) and isoflurane (GABAergic and non-GABAergic targets) on fear-conditioned learning and θ oscillations in freely behaving rats.

RESULTS

All three anesthetics slowed θ peak frequency in proportion to their inhibition of fear conditioning (by 1, 0.7, and 0.5 Hz for 0.32% isoflurane, 60% N2O, and 0.24% halothane, respectively). Anesthetics inconsistently affected other characteristics of θ oscillations.

CONCLUSIONS

At subhypnotic amnestic concentrations, θ-oscillation frequency was the parameter most consistently affected by these three anesthetics. These results are consistent with the hypothesis that modulation of the θ rhythm contributes to anesthetic-induced amnesia.

Protective effects of volatile agents against acetylcholine-induced bronchoconstriction in isolated perfused rat lungs

BACKGROUND

Bronchoactive properties of volatile agents against lung constriction are well established. The purpose of this study was to investigate the ability of halothane (Hal), isoflurane (Iso), sevoflurane (Sev) and desflurane (Des) to alter the lung mechanics in the absence of an airway tone and during acetylcholine (Ach)-induced bronchoconstriction.

METHODS

Low-frequency pulmonary impedance data (ZL) were collected from isolated, normo-perfused rat lungs under baseline conditions and following the injection of Ach (0.1 mg/kg) into the pulmonary artery. Measurements were performed without the administration of any anaesthetic agent in the first phase of the experiments and during inhalation without any volatile agent (control group, n = 6) or during inhalation of Hal (n = 6), Iso (n = 9), Sev (n = 6) or Des (n = 8) at 1 minimum alveolar concentration (MAC). The airway resistance (Raw) and parenchymal damping and elastance were estimated from the Z(L) data by model fitting.

RESULTS

Under baseline conditions, the basic value of Raw was significantly decreased by Des (- 31.2 +/- 3.8%) and Sev (- 18.0 +/- 4.5%) administration, whereas Hal and Iso did not have a statistically significant effect on Raw (- 3.3 +/- 5.1% and - 8.6 +/- 2.4%, respectively). Moreover, all four inhalation anaesthetics prevented the increase in Raw following Ach administration, the findings ranging between - 14.3 +/- 11.4% for Hal and - 37.5 +/- 10.9% for Sev.

CONCLUSIONS

Our results on a denervated isolated perfused lung model demonstrate the potential of Des and Sev to decrease the basal airway tone, whereas Iso and Hal are ineffective in this regard. All of these volatile agents markedly protect against Ach-induced bronchoconstriction.

Genetic background differences between FVB and C57BL/6 mice affect hypnotic susceptibility to pentobarbital, ketamine and nitrous oxide, but not isoflurane

BACKGROUND

Pharmacogenomics has allowed us to identify the mechanisms underlying much of the inherited variability in drug response. There have been several reports of strain-dependent anesthetic actions in rodents, indicating that significant genetic differences exist in the hypnotic and antinociceptive effects of various anesthetics.

METHODS

Loss of righting reflex was used to compare the hypnotic action of pentobarbital, ketamine, nitrous oxide and isoflurane between two genetically different populations of mice, C57BL/6 with black hair and Friends virus B (FVB) with white hair.

RESULTS

C57BL/6 mice were more susceptible than FVB mice to the hypnotic effects of ketamine, pentobarbital and nitrous oxide. However, the sensitivity to isoflurane did not differ between C57BL/6 and FVB mice.

CONCLUSION

Genetic background affects the hypnotic susceptibility to some anesthetic agents in mice. Our results indicate that there may be a different genetic basis for the operation of hypnosis between isoflurane and other anesthetics, such as pentobarbital, ketamine and nitrous oxide.