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Anesth Analg 2000;91:1013-1016
© 2000 International Anesthesia Research Society

GENERAL ARTICLES

The Nonimmobilizer 1,2-Dichlorohexafluorocyclobutane Does Not Affect Thermoregulation in the Rat

Anya J. Maurer, BS, Daniel I. Sessler, MD, Edmond I Eger, II, MD, and James M. Sonner, MD

Department of Anesthesia and Perioperative Care, University of California, San Francisco, California

Address correspondence to James M. Sonner, MD, Department of Anesthesia, 513 Parnassus Ave., Box 0464, University of California, San Francisco, CA 94143-0464. Address e-mail to sonnerj{at}anesthesia .ucsf.edu.


   Abstract
Top
 Abstract
Introduction
 Methods
 Results
 Discussion
 References
 
Inhaled and other anesthetics profoundly affect the central nervous system, causing amnesia, immobility in the face of noxious stimulation, and depression of thermoregulation. Nonimmobilizers, inhaled compounds whose lipophilicity suggests that they should be anesthetics, do not produce immobility, but they do cause amnesia. Their effects on thermoregulation were the subject of the present study. We gave eight rats isoflurane on one occasion and the nonimmobilizer 2N (1,2-dichlorolhexafluorocyclobutane) on another. We measured the effect of various concentrations of each compound on thermoregulation provoked by body cooling. The specific outcome was increased metabolism, as reflected in increased output of carbon dioxide. Isoflurane decreased the temperature threshold for such increases and the maximum response intensity, doing so in a concentration-dependent manner, whereas 2N had a minimal or no effect at any concentration up to 0.9 minimum alveolar concentration (estimated from its lipophilicity). Thus, 2N may be a useful tool for studies of the mechanisms mediating the thermoregulatory depression produced by anesthetics: 2N should not affect such a mechanism.

Implications: Unlike inhaled anesthetics, the nonimmobilizer 2N (1,2-dichlorohexafluorocyclobutane) minimally affects temperature regulation in rats. Thus, 2N may be useful in mechanistic studies of temperature regulation. Cellular and molecular sites that mediate the capacity of inhaled anesthetics to depress thermoregulation should be influenced by anesthetics but not by 2N.


   Introduction
Top
 Abstract
 Introduction
Methods
 Results
 Discussion
 References
 
Inhaled anesthetics have predictable effects on the central nervous system, including immobility in response to surgical stimuli (1), amnesia (2), altered nociceptive thresholds (3), and impaired thermoregulatory responses (4). Nonimmobilizers (previously called nonanesthetics) are inhaled compounds expected from the Meyer-Overton hypothesis to have anesthetic properties. According to that relationship, the product of MAC (the minimum alveolar concentration of anesthetic required to abolish movement in response to noxious stimuli in 50% or subjects) and lipid solubility (measured by the oil/gas partition coefficient) is a constant. Nonimmobilizers, however, do not prevent movement in response to noxious stimuli (5), including thermal stimulation (i.e., tail flick latency) (3). Anesthetic-induced immobility and depression of tail-flick latency are both effects on spinal reflexes. Although they do not affect these reflexes, nonimmobilizers impair a higher function of the central nervous system, namely learning and memory. For example, nonimmobilizers interfere with fear conditioning at partial pressures predicted by lipophilicity to be anesthetic (6).

Because they share some anesthetic properties, but not others, nonimmobilizers are potentially useful probes of the relevance of putative targets of volatile anesthetics and have been used as controls in biochemical systems characterizing anesthetic mechanisms. For example, a receptor important to the immobilizing effect of anesthetics should be sensitive to an inhaled anesthetic but not respond to a nonimmobilizer. Likewise, a receptor important to the effect of anesthetics on fear conditioning should be affected similarly by both anesthetics and nonimmobilizers.

In humans, anesthetics decrease vasoconstriction and shivering thresholds (triggering core temperatures) and increase sweating thresholds, thus increasing the interthreshold range (temperatures not triggering autonomic thermoregulatory defenses) (7). All anesthetics so far tested markedly increase the interthreshold range (8–9). The gain of thermoregulatory responses (increment in response intensity with deviation of core temperature beyond the triggering threshold) remains unchanged with some anesthetics (10–11) but is decreased by others (12). Similarly, maximum response intensity of some responses remains normal (10,12), whereas maximum intensity in other cases is decreased (13). However, all anesthetics so far tested at the very least markedly increase the interthreshold range.

The thermoregulatory effects of nonimmobilizers are unknown. We therefore tested the hypothesis that the nonimmobilizer 1,2-dichlorohexafluorocyclobutane (hereafter called 2N) does not reduce the threshold, gain, or maximum intensity of nonshivering thermogenesis.


   Methods
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 Abstract
 Introduction
 Methods
Results
 Discussion
 References
 
With the approval of the Committee on Animal Research of the University of California, San Francisco, we studied eight male Sprague-Dawley rats (Crl:CD(SD)BR) from Charles River Laboratories (Hollister, CA) weighing 312–564 g The animals were housed up to three per cage under 12:12 h light:dark cycles and supplied with rat chow and water ad libitum.

Four animals were studied at a time. Animals were first studied with isoflurane and then on a later day with 2N. A colonic temperature probe (YSI Inc., Yellow Springs, OH) was inserted into each rat for evaluation of core temperature. The rats were each positioned in a gas-tight PlexiglasTM (McMaster-Carr, Los Angeles, CA) tube 8.5 cm in internal diameter by 40 cm in length. Throughout the study, gas flow was maintained at 1 L/min with calibrated flow meters (Air Products, Allentown, PA). Only oxygen was delivered to the chamber for a 30-min long equilibration period. Subsequently, either isoflurane (Baxter PPD, Liberty Corner, NJ) at 0.17, 0.34, 0.51, 0.69, or 0.86 MAC corresponding to 0.25%, 0.50%, 0.75%, 1.00%, or 1.25% atmospheres partial pressure respectively, or 2N (PCR Inc., Gainesville, FL) at 0.30, 0.60, or 0.9 predicted MAC corresponding to 1.25%, 2.50%, or 3.75% atmospheres partial pressure, respectively, was added to the oxygen using conventional anesthesia vaporizers. MAC for isoflurane was taken as 1.45% atmospheres partial pressure. MAC for 2N was calculated from its oil/gas partition coefficient of 43.5 (5) and the Meyer-Overton relationship that MAC x (oil/gas partition coefficient) = 1.82 (14), giving a predicted MAC of 4.2% atmospheres partial pressure. Mixed expired carbon dioxide for each animal was measured by capnography (Ohmeda model 5250 capnograph, Louisville, CO).

Rats were studied at each dose of each inhaled compound for 2 h. Ambient temperature was 22°C. During the first hour, baseline measurements of rectal temperature and mixed expired carbon dioxide were made every 10 min. After 1 h, plastic bags filled with ice were applied around the PlexiglasTM chambers for an additional hour, to trigger thermoregulatory responses to hypothermia. Measurements of carbon dioxide and temperature were continued every 10 min until the end of study. Isoflurane and 2N concentrations were analyzed by using gas chromatography (Gow-MAC Instrument Corp., Bridgewater, NJ). No animal was allowed to get colder than 32.0°C.

Three parameters were determined by three blinded observers using plots of mixed expired carbon dioxide versus time (after a 30-min equilibration period) for every animal at each dose of study compound. The parameters were 1) the minimum carbon dioxide plateau; 2) the maximum carbon dioxide plateau; and 3) the first point at which there was a sustained increase in carbon dioxide above the minimum. From these parameters, three thermoregulatory parameters were derived: 1) the thermoregulatory threshold temperature, which was the temperature at which there was a sustained increase in carbon dioxide; 2) the gain (in mm Hg/degree centigrade), which was calculated by using least squares linear regression of the points connecting the maximum and minimum carbon dioxide plateaus; and 3) the maximum response, which was defined by the ratio of the maximum carbon dioxide plateau to the minimum carbon dioxide plateau (Fig. 1).



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  		Figure 1. Mixed expired carbon dioxide as a function of time. In this idealized study, a rat exposed to isoflurane had a 5 mm Hg mixed expired carbon dioxide partial pressure before cooling at 60 min. Cooling provoked a thermoregulatory response, which led to an increased metabolic rate reflected by an increase in mixed expired carbon dioxide. The thermoregulatory threshold was the temperature at the first point after cooling that was part of a sustained increase above the baseline carbon dioxide response. The maximum carbon dioxide response is the plateau response shown during cooling. The gain is the slope of the regression line connecting the minimum and maximum carbon dioxide plateaus. The maximum intensity of the response is the ratio of the maximum and minimum carbon dioxide plateau responses.

 
The thermoregulatory responses for each animal at each dose of study drug were taken as the median value from the blinded observers. The thermoregulatory parameters for each drug at its various doses were compared by using one-way analysis of variance and a Student-Newman-Keuls test. P < 0.05 was considered statistically significant.


   Results
Top
 Abstract
 Introduction
 Methods
 Results
Discussion
 References
 
Baseline core temperatures were comparable before administration of isoflurane versus 2N. Isoflurane produced a concentration-dependent reduction in the threshold for nonshivering thermogenesis, from 38.6°C (oxygen only) to the cutoff value of 32.0°C (at 0.86 MAC isoflurane) (P < 0.001). In contrast, 2N did not significantly decrease the threshold except at the highest partial pressure tested (0.9 MAC predicted) (P = 0.021 for analysis of variance, with P < 0.05 by Student-Newman-Keuls test for comparison of the oxygen control group with that receiving 0.9 MAC predicted of 2N). At 0.9 predicted MAC of 2N, the threshold was 37.3°C. This decline of 1.3°C from control animals breathing oxygen was one-fifth the 6.6°C decrease seen with a comparable MAC fraction of isoflurane (Fig. 2).



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  		Figure 2. Isoflurane (Iso.) dose dependently reduces thermoregulatory thresholds; the nonimmobilizer 1,2-dichlorohexafluorocyclo- butane (2N) does not. MAC = minimum alveolar concentration.

 
Isoflurane also decreased the maximal carbon dioxide response (Fig. 3) (P < 0.001). However, unlike the threshold for nonshivering thermogenesis, the decrease was only significant (P < 0.05) at isoflurane concentrations exceeding 0.34 MAC. 2N did not affect the maximal carbon dioxide response (P = 0.28). No concentration of isoflurane (P = 0.80) or 2N (P = 0.54) affected the gain.



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  		Figure 3. Isoflurane (Iso.) reduces the maximum carbon dioxide response to cooling above 0.34 MAC. The nonimmobilizer 1,2-dichlorohexafluorocyclobutane (2N) does not affect the intensity of the carbon dioxide response to cooling. MAC = minimum alveolar concentration.

 

   Discussion
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
References
 
Homeothermic species tightly regulate core body temperature. In humans, the normal range of core temperatures, as defined by the sweating-to-vasoconstriction interthreshold range, is only about 0.2°C (15). In adults, the first autonomic response to hypothermia is arteriovenous shunt vasoconstriction. In contrast, shivering is observed only when body temperature decreases an additional 1°C (16). Rodents and other small animals use nonshivering thermogenesis instead of shivering as their primary defense against hypothermia (17). Nonshivering thermogenesis is therefore the thermoregulatory defense most commonly evaluated in rodent studies, and that is the response we tested.

The thermoregulatory effects of anesthetics and sedatives are far better characterized in humans than experimental animals. Isoflurane decreases the threshold for vasoconstriction and shivering in humans, 0.9 MAC decreasing these thresholds approximately 4.5°C (9). A similar MAC-fraction of isoflurane produced an equal or greater decrease in the nonshivering thermogenesis threshold in rats (6.6°C) even though volatile anesthetics peripherally impair nonshivering thermogenesis (18). Thus, anesthetics profoundly impair thermoregulatory response thresholds in both species.

The effects of isoflurane on the vasoconstriction threshold were initially thought to be rectilinear. However, subsequent research showed disproportionately greater decreases in vasoconstriction and shivering thresholds produced by higher anesthetic partial pressures of isoflurane (9) or desflurane (19). In our study, isoflurane appeared to linearly decrease the nonshivering thermogenesis threshold in rats (Fig. 2), although one might imagine a curving downward at the highest applied concentration of isoflurane (and recall that we stopped the study at 32°C and thus underestimated the decrease at this highest concentration). Whether this is a general feature of thermoregulatory responses in rodents or specific to nonshivering thermogenesis remains to be determined.

In humans, isoflurane anesthesia significantly reduces the maximum intensity of shivering (13). This result is consistent with our current observation that concentrations exceeding 0.34 MAC progressively decreased the maximum response intensity in rats, halving the response at approximately 0.9 MAC. Thus, in rats, isoflurane markedly decreased the threshold for nonshivering thermogenesis and the maximum intensity of this response.

In contrast, the nonimmobilizer 2N minimally affected the threshold and maximum intensity of nonshivering thermogenesis, and then only at the highest test concentration. That is, isoflurane profoundly impaired thermoregulation whereas 2N did not. The nonimmobilizer 2N does not affect thermoregulation. This lack of effect parallels its actions on mobility and nociception. Such results differ from the concentration-dependent capacity of 2N to impair learning and memory as measured by fear conditioning.

This has practical consequences for animal studies involving 2N. Core temperature tends to decline in animals exposed to anesthetics. Because this can interfere with behavioral measurements [e.g., MAC declines as core temperature decreases (20)] maintenance of normothermia is essential in animal studies of anesthetic action. This is only a minor issue with the nonimmobilizer 2N, because thermoregulatory defenses remain intact.

Hypothermia is an undesirable side effect of general anesthetics. The minimal or absent capacity of 2N to depress thermoregulatory mechanisms means that it may serve as a tool in studies of mechanisms by which anesthetics depress these mechanisms. A putative site that mediates the depression produced by anesthetics should be influenced by anesthetics but not by 2N. Whether other nonimmobilizers also have minimal effects on thermoregulation remains to be tested.

In summary, the nonimmobilizer 2N has essentially no effects on thermoregulation. Isoflurane strongly inhibits thermoregulation in rats by reducing the thermoregulatory threshold and response intensity.


   Acknowledgments
 
This work was supported, in part, by National Institutes of Health Grants 1P01GM47818 and GM58273, and the Joseph Drown Foundation.


   Footnotes
 
This work, in part, has been submitted for presentation at the Annual Meeting of the American Society of Anesthesiologists, San Francisco, CA, October 2000.

EIE, II, is a paid consultant to Baxter Pharmaceutical Products, Inc.


   References
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 

  1. Eger EI. Anesthetic Uptake and action. Baltimore: Williams and Wilkins, 1974.
  2. Chortkoff B, Bennett H, Eger EI. Subanesthetic concentrations of isoflurane suppress learning as defined by the category-example task. Anesthesiology 1993; 79: 16–22.[ISI][Medline]
  3. Sonner J, Li J, Eger EI. Desflurane and nitrous oxide, but not nonimmobilizers, affect nociceptive responses. Anesth Analg 1998; 86: 629–34.[Abstract]
  4. Sessler DI. Perianesthetic thermoregulation and heat balance in humans. FASEB J 1993; 7: 638–44.[Abstract]
  5. Koblin DD, Chortkoff BS, Laster MJ, et al. Polyhalogenated and perfluorinated compounds that disobey the Meyer-Overton hypothesis. Anesth Analg 1994; 79: 1043–8.[Abstract/Free Full Text]
  6. Kandel L, Chortkoff BS, Sonner J, et al. Nonanesthetics can suppress learning. Anesth Analg 1995; 82: 321–6.[Abstract]
  7. Sessler DI. Perioperative hypothermia. N Engl J Med 1997; 336: 1730–7.[Free Full Text]
  8. Matsukawa T, Kurz A, Sessler DI, et al. Propofol linearly reduces the vasoconstriction and shivering thresholds. Anesthesiology 1995; 82: 1169–80.[ISI][Medline]
  9. Xiong J, Kurz A, Sessler DI, et al. Isoflurane produces marked and non-linear decreases in the vasoconstriction and shivering thresholds. Anesthesiology 1996; 85: 240–5.[ISI][Medline]
  10. Washington D, Sessler DI, Moayeri A, et al. Thermoregulatory responses to hyperthermia during isoflurane anesthesia in humans. J Appl Physiol 1993; 74: 82–7.[Abstract/Free Full Text]
  11. Ikeda T, Sessler DI, Tayefeh F, et al. Meperidine and alfentanil do not reduce the gain or maximum intensity of shivering. Anesthesiology 1998; 88: 858–65.[ISI][Medline]
  12. Kurz A, Xiong J, Sessler DI, et al. Desflurane reduces the gain of thermoregulatory arterio-venous shunt vasoconstriction in humans. Anesthesiology 1995; 83: 1212–9.[ISI][Medline]
  13. Ikeda T, Kim J-S, Sessler DI, et al. Isoflurane alters shivering patterns and reduces maximum shivering intensity. Anesthesiology 1998; 88: 866–73.[ISI][Medline]
  14. Taheri S, Halsey M, Liu J, et al. What solvent best represents the site of action of inhaled anesthetics in humans, rats, and dogs? Anesth Analg 1991; 72: 627–34.[Abstract/Free Full Text]
  15. Lopez M, Sessler DI, Walter K, et al. Rate and gender dependence of the sweating, vasoconstriction, and shivering thresholds in humans. Anesthesiology 1994; 80: 780–8.[ISI][Medline]
  16. Kurz A, Go JC, Sessler DI, et al. Alfentanil slightly increases the sweating threshold and markedly reduces the vasoconstriction and shivering thresholds. Anesthesiology 1995; 83: 293–9.[ISI][Medline]
  17. Foster DO, Frydman ML. Nonshivering thermogenesis in the rat. II. Measurements of blood flow with microspheres point to brown adipose tissue as the dominant site of the calorigenesis induced by noradrenaline. Can J Physiol Pharmacol 1978; 56: 110–22.[ISI][Medline]
  18. Dicker A, Ohlson KB, Johnson L, et al. Halothane selectively inhibits nonshivering thermogenesis. Anesthesiology 1995; 82: 491–501.[ISI][Medline]
  19. Annadata RS, Sessler DI, Tayefeh F, et al. Desflurane slightly increases the sweating threshold, but produces marked, non-linear decreases in the vasoconstriction and shivering thresholds. Anesthesiology 1995; 83: 1205–11.[ISI][Medline]
  20. Eger EI, Johnson B. MAC of I-653 in rats, including a test of the effect of body temperature and anesthetic duration. Anesth Analg 1987; 66: 974–6.[Abstract/Free Full Text]
Accepted for publication June 5, 2000.




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Lippincott, Williams & Wilkins Anesthesia & Analgesia® is published for the International Anesthesia Research Society® by Lippincott Williams & Wilkins with the assistance of Stanford University Libraries' HighWire Press®. Copyright 2006 by the International Anesthesia Research Society. Online ISSN: 1526-7598   Print ISSN: 0003-2999 HighWire Press