Application of Dixon's Up-and-Down Design to Estimate the Minimum Alveolar Concentration of Sevoflurane in Rats with Refined Movement Classification

Summary

The protocol presented herein seeks to evaluate the depth of anesthesia utilizing Dixon's Up-and-Down Design in sedated rats subjected to a standardized pain stimulus. Furthermore, it delineates a classification of the various movement patterns observed.

Abstract

When using volatile anesthetic agents, the standard measure of potency is the minimum alveolar concentration (MAC), defined as the alveolar concentration at which 50% of subjects do not show a motor response to a painful stimulus. In clinical and experimental studies, the current consensus states that only "gross purposeful movements" count as a positive motor response and can, therefore, indicate too mild sedation. However, the appropriate level of anesthesia, in both humans and animals, varies individually based on various influencing factors, such as other medications administered. Two animal studies in rats were conducted to examine the effects of tetrahydrocannabinol and ethanol on sevoflurane, one of the most commonly used volatile anesthetics. To estimate the MAC, Dixon's Up-and-Down Design was employed. For the first animal, a predetermined dose of sevoflurane was chosen. After an equilibration period of 15 min, a standardized pain stimulus was applied to the distal third of the rat's tail using a surgical clamp. The animal's response to the stimulus was documented as positive or negative. If a positive motor reaction occurred, the dose for the next animal was increased. Conversely, if the rat did not exhibit a motor response, the dose for the following animal was decreased. Isotonic regression and the bootstrap method were used to estimate the MAC and 95% confidence intervals. This article outlines a detailed protocol for assessing anesthetic depth in rats using the up-and-down-design. It also provides a novel and detailed description and categorization of movement patterns in sedated rats when a standardized painful stimulus is applied to enhance reproducibility for future studies.

Introduction

Sevoflurane is one of the most frequently used volatile inhaled anesthetics¹. Its potency is assessed using the minimum alveolar concentration (MAC), which is defined as the alveolar concentration at which 50% of patients do not show a motor response to a painful stimulus^2,3,4. To establish the MAC, rats can be used as animal models due to their greater homogeneity compared to humans. The pain stimulus is typically applied using a surgical clamp on the distal third of the rat's tail². Using alternative methods for pain testing would not guarantee comparability with previously conducted studies and could, therefore, generate divergent results. Due to the great variety of movement patterns in response to the stimulus, the current consensus states that only "gross purposeful movements" are to be considered positive motor reactions³. However, this definition lacks further refinement regarding which specific categories would qualify as "gross purposeful" and to what extent a movement should meet this criterion. This is of utmost importance, as a motor response to a stimulus can indicate, in a clinical setting, insufficient depth of sedation.

Various factors influence the anesthetic state, including age⁵, comorbidities⁶, impaired liver function⁷ or hypothermia⁸. Moreover, the medications administered along with anesthetics significantly affect the depth of anesthesia. For instance, fentanyl and propofol, often used during the induction phase, reduce the required amount of sevoflurane^9,10. Additionally, it has been shown that lidocaine or pregabalin can lower the MAC of sevoflurane^11,12. Furthermore, recent intake of alcohol or substances such as tetrahydrocannabinol (THC) can also reduce MAC, thus enhancing the depth of anesthesia^13,14. Conversely, it has been suggested that sympathomimetic agents may lighten sedation by stimulating the central nervous system¹⁵.

To enhance reproducibility for future studies, a detailed protocol and video descriptions for assessing the MAC in rats using Dixon's up-and-down design¹⁶ are provided. In contrast to previous MAC studies, this protocol outlines detailed procedural steps to support reproducibility across laboratories. Furthermore, video footage from two previously published studies was reanalyzed, and motor responses were classified based on their extent and pattern (coordinated movements - rolling, stretching, saltatory, and uncoordinated movement curling). Therefore, this study also offers an innovative and thorough description and classification of movement patterns in sedated rats in response to a standardized painful stimulus, aiming to improve reproducibility for future research.

Protocol

All experimental animal studies have been approved by the Intramural Committee for Animal Experimentation of the Medical University of Vienna and the competent regulatory authority (Austrian Federal Ministry of Women, Science and Research). The experiments were conducted under constantly maintained anesthesia. The animals do not consciously perceive pain even if a movement should occur¹⁷. Adult Wistar rats at the ages of 81 to 95 days were used.

1. Study animals and housing

1. House the animals according to weight and sex. Separate males and females and house them for a minimum of 14 days before starting the experiment to allow acclimatization. Place a maximum of three rats over 400 g or four rats under 400 g per cage.
2. Maintain environmental conditions with a 12-h light/dark cycle, ambient temperature of 22 °C, and humidity levels between 45% and 65%.
3. Weigh each animal regularly and record body weight before each procedure.
4. Transport animals individually from the housing area to the testing facility. Cover each cage with a cloth to reduce visual stress, and house untested animals in a separate room to avoid exposure to experimental procedures.

2. Equipment setup

1. Verify that all required equipment is functional and available before starting the experiment. Perform equipment checks according to institutional guidelines or standard operating procedures, including leak testing, calibration, and power function tests where applicable.
2. Use an airtight, transparent chamber (15 cm × 20 cm × 9 cm) for placing the rat. Ensure the chamber is equipped with a 15 mm opening to externally guide the tail while minimizing gas leakage.
3. Connect the inflow line to a sevoflurane vaporizer and an oxygen source.
4. Connect the outflow line on the opposite side of the chamber to both a gas extraction system and an anesthesia monitor to record sevoflurane concentrations.
5. Confirm that the rectal thermometer, heating blanket, and infrared heating lamp are operational. Test the thermometer against a reference device, check that the heating blanket reaches target temperature ranges, and verify that the infrared lamp turns on and maintains stable output. Position the lamp directly above the testing chamber at a safe distance in accordance with local safety procedures.
6. Prepare a 25 cm arterial clamp to be used for the standardized pain stimulus.

3. Drug preparation

1. THC preparation (optional)
   1. Use a THC formulation with a concentration of 25 mg/mL. Calculate the required volume for each animal by multiplying the body weight in kilograms by (10 mg/kg/25 mg/mL), resulting in 0.4 mL per kg of body weight.
   2. Use a positive displacement pipette to draw up the exact calculated volume for each animal. Do not administer at this stage. Prepare the pipette and set it aside for later administration during the drug administration step (see step 4.6).
   3. Use a pipette tip suitable for gavage, such as one that can safely hold up to 0.3 mL of fluid. Perform the oral gavage directly using the loaded pipette. Discard the pipette tip after each use to prevent cross-contamination.
2. Ethanol preparation (optional):
   1. Prepare ethanol at the required dosage concentration of 1 g/kg or 2 g/kg. Use a highly concentrated formulation of ethanol (e.g., 96-100%) as the base and dissolve it in 0.9% saline to a total volume of 10 mL/kg body weight for each individual animal. Use the same volume of saline alone as the corresponding placebo. Follow all institutional safety guidelines, particularly those related to the handling and storage of flammable substances.
   2. Use an adequate syringe (5 mL or 10 mL, depending on the animal's weight) to draw up the exact calculated volume for each individual animal. Do not administer the substance at this stage. Prepare the syringe and set it aside for intraperitoneal injection during the drug administration step (see step 4.6). Attach a 23 G needle for administration and cover it with a corresponding cap to prevent injury until use.

4. Drug application

1. Induce anesthesia for drug administration by placing the rat in the chamber and flooding it with 3.5 vol% sevoflurane in 100% oxygen at 3 L/min.
2. Wait approximately 3-5 min until the animal is visually unconscious.
3. Perform the righting reflex test by gently tilting the chamber to roll the animal onto its back. If it does not attempt to right itself within 10 s, assume adequate depth of anesthesia.
4. Temporarily reduce sevoflurane concentration to 0 and flush the chamber with 100% oxygen for 15 s to minimize anesthetic gas exposure for the experimenter.
5. Remove the animal from the chamber and place it in front of a gas delivery tube delivering the same sevoflurane concentration to maintain anesthesia during administration. Skip this step if the administration of the test substance is brief enough to avoid recovery from anesthesia.
6. Administer substances per experimental design: Deliver 10 mg/kg THC via gastric gavage or administer 1 g/kg or 2 g/kg ethanol intraperitoneally. Refer to Section 3 for preparation details and follow institutional guidelines for performing both gavage and intraperitoneal administration procedures.
7. Return the animal to its cage to recover.
8. Wait for 1 h for ethanol and 2 h for THC before continuing with the procedure.

5. Anesthesia induction for MAC testing

1. After the waiting period, place the rat back in the airtight testing chamber and flood it with the determined mixture of sevoflurane and oxygen. Ensure a constant oxygen flow of 3 L/min. Determine the individual sevoflurane concentration (step 9.1) for each animal, starting with 2.6 vol% (or another predetermined starting point based on statistical planning) for the first animal in each group. A detailed description of the Up-and-Down-Design is given below.
2. Turn the infrared heating lamp on.
3. Wait approximately 3-5 min until the animal is visually unconscious.
4. Perform the righting reflex test by gently tilting the chamber to roll the animal onto its back. If it does not attempt to right itself within 10 s, assume adequate depth of anesthesia.
5. Temporarily reduce sevoflurane concentration to 0 and flush the chamber with 100% oxygen for 15 s to minimize anesthetic gas exposure for the experimenter.
6. Quickly open the chamber and insert the rectal thermometer. Monitor core body temperature throughout the procedure and adjust the intensity of the infrared heating lamp and the heating blanket accordingly.
7. Lead the tail outside via the small opening in the chamber.
8. Close the chamber quickly.
9. Immediately reestablish the desired sevoflurane concentration before proceeding.

6. Equilibration time

1. Allow a period of 15 min for equilibration.
2. Continually monitor the animal's core temperature (37.2 °C ± 0.3 °C) and adjust the heating devices accordingly to maintain normothermia.

7. Pain stimulus

1. Identify the distal third of the tail.
2. Apply the pain stimulus by locking the clamp in its third position (approximately 50 N of force).
3. Leave the clamp locked in its third position until the rat shows a positive movement response but for a maximum of 1 min.
4. Record in the protocol the presence or absence of a motor reaction. Provide a brief description of the observed movement in terms of its pattern and extent. Refer to point 11 for the classification of movement patterns.

8. Recovery

1. Turn off the vaporizer and the heating lamp and place the rat back in the cage.

9. Up-and-down design

1. Determine the sevoflurane concentration for the following experiment using Dixon's Up-and-Down-Design. If the previous animal showed a motor response to the stimulus, increase the concentration for the next animal by 0.2%pt. Conversely, if the previous animal did not show any motor response, decrease the concentration by 0.2%pt.

10. Euthanasia (if applicable)

NOTE: Euthanasia is not inherently a necessary part of the experiments. The competent Intramural Committee for Animal Experiments must be consulted. For the two referenced studies, it was decided to perform intracardiac blood draws and euthanize the animals afterward. The animals were not returned to the cage as described in step 8.1, but instead, the procedure continued directly with step 9.1.

1. Prepare the euthanasia solution by dissolving 300 mg of pentobarbital in 1 mL of saline. Draw the solution into a syringe and keep it ready for immediate use following the blood draw.
2. Take the rat out of the box and place it in front of a breathing tube delivering 3.5 vol% sevoflurane in 100% oxygen to maintain constant anesthesia. Position the animal on its back to allow unobstructed access to the thoracic area.
3. Perform the intracardiac blood draw using a 23 G needle and a 5 mL syringe. Gently palpate the heartbeat with a finger to locate the heart. Insert the needle just caudal to the xiphoid process, slightly left of midline and at a shallow angle.
4. Aspirate gently while adjusting the needle position until blood flow is observed. Collect the required volume.
5. After completing the blood draw, carefully remove the syringe from the needle and attach the preloaded syringe containing the pentobarbital solution. Administer the entire 1 mL of the solution intracardially to ensure euthanasia in accordance with institutional ethical guidelines.

11. Continuing the experiments

1. Repeat the experiment starting from step 3 for each study animal individually.

12. Classification of movement patterns (optional)

1. Classify behavioral responses into the following categories:
   1. Rolling: Classify as rolling when the behavioral response involves an axial rotation of the body, typically along the longitudinal axis.
   2. Stretching: Classify as stretching when the behavioral response is characterized by a bilateral, symmetric outward extension of the limbs and body.
   3. Saltatory movement: Classify as saltatory movement when the behavioral response consists of sudden, hindlimb-focused kicks or jerks resembling short, hopping-like bursts of the whole body.
   4. Curling: Classify as curling when the behavioral response involves uncoordinated, inward-directed movement of varying intensity, causing the body to bend toward its center.

Results

In the THC study, the bootstrap estimate of the MAC was 2.8 vol% in the control group (95% CI: 2.7-2.9 vol%) and 2.1 vol% in the 10 mg/kg THC group (95% CI: 1.8-2.4 vol%)¹³.

In the ethanol study, the estimated MAC was 2.24 vol% in the control group (95% CI: 1.97-2.94 vol%). In the 1 g/kg ethanol group, it was 1.65 vol% (95% CI: 1.40-1.98 vol%), and in the 2 g/kg ethanol group, 1.08 vol% (95% CI: 0.73-1.42 vol%)¹⁴. The differing number of decimal ...

Discussion

This study refines the protocol for determining the MAC of sevoflurane in rats by detailing critical methodological steps and addressing common challenges to contribute to a more standardized and reproducible approach for assessing anesthetic depth.

Accurate MAC determination is essential for both experimental and clinical anesthetic practices. This work contributes to the existing literature by providing a clear and reproducible method for assessing sevoflurane potency in animal models. The r...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Airtight transparent chamber	N/A	N/A	Generic
Arterial clamp	KLS Martin	13-410-22-07	25 cm
Dräger Primus	Dräger Austria GmbH	Primus
Dräger Vapor 2000	Dräger Austria GmbH	Vapor 2000
Homeothermic Blanket Control Unit	Harvard Apparatus, Holliston, Massachusetts, USA	50-7137	230 V
Manual Mobile Infant Warmer	Fisher/Paykle, Auckland, New Zealand	IW9210/920