Anesth Analg 2003;96:1661-1664
© 2003 International Anesthesia Research Society
ANESTHETIC PHARMACOLOGY
-2 Adrenoreceptors Probably Do Not Mediate the Immobility Produced by Inhaled Anesthetics
Edmond I Eger, II, MD,
Yilei Xing, MD,
Michael J. Laster, DVM, and
James M. Sonner, MD
Department of Anesthesia and Perioperative Care, University of California, San Francisco
Address correspondence and reprint requests to Edmond I Eger II, MD, Department of Anesthesia, S-455, University of California, San Francisco, CA 94143-0464. Address e-mail to egere{at}anesthesia ucsf.edu.
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Abstract
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Agonism of 
-adrenoreceptors has a powerful anesthetic result mediated, in part, by effects on the spinal cord. -adrenoreceptor agonists (e.g., dexmedetomidine) can decrease the minimum alveolar anesthetic concentration (MAC) of inhaled anesthetics (e.g., halothane) to zero, with an apparently additive interaction between halothane and dexmedetomidine. We tested whether the capacity of the inhaled anesthetic isoflurane to produce immobility in the face of noxious stimulation resulted from agonism of -adrenoreceptors. MAC (the concentration required to eliminate movement in response to a noxious stimulus in 50% of subjects) of isoflurane was determined before and after intraperitoneal administration of the -adrenoreceptor antagonists yohimbine and atipamezole. The doses of yohimbine and atipamezole equaled or exceeded those that reverse the ability of agonism of -adrenoreceptors to decrease MAC. Smaller doses of yohimbine or atipamezole slightly increased (by 10%) the MAC of isoflurane, an increase we interpret as the result of blockade of a small amount of tonically active -adrenoreceptor activity. Doses five-fold larger did not change MAC. Doses 10-fold larger decreased MAC. We conclude that -adrenoreceptors do not or minimally mediate the capacity of inhaled anesthetics to produce immobility.
IMPLICATIONS: Although stimulation (agonism) of -2 adrenoreceptors can decrease the inhaled anesthetic concentration required to produce immobility in the face of noxious stimulation, blockade of -2 adrenoreceptors minimally affects the concentration. Thus, augmentation of the effect of -2 adrenoreceptors is not an appreciable part of the mechanism whereby inhaled anesthetics produce immobility.
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Introduction
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" -2 adrenoreceptor activity is involved in the mechanism of anesthesia (1)." So begins a report published by Kushikata et al. (1), one that shows that clonidine (an -2 adrenoreceptor agonist) increases propofol-induced sleep time, whereas yohimbine (an -2 adrenoreceptor antagonist) decreases sleep time. Yohimbine can antagonize some anesthetic effects of the popular veterinary anesthetic xylazine (2), doing so at doses an order of magnitude less than the lethal dose of yohimbine, approximately 20 mg/kg. Yohimbine can also antagonize the analgesic effects of morphine (3).
More immediately pertinent to the present study, several reports find that the administration of -2 adrenoreceptor agonists can decrease the MAC (the minimum alveolar concentration required to suppress movement in response to noxious stimulation in 50% of subjects) of inhaled anesthetics in rats (4,5) and dogs (6,7). And clonidine decreases anesthetic requirement in humans (8–10).
The locus coeruleus mediates a major portion of the hypnotic effect of -2 adrenoreceptor agonists (11–13). The effects of -2 adrenoreceptor agonists may result from inhibition of ion conductance through L- or P-type calcium channels and/or facilitation of conductance through voltage-gated or calcium-activated potassium channels (14), possibly through inhibition of adenylate cyclase (15). In contrast to their effects on the locus coeruleus, -2 adrenoreceptor agonists have minimal effects on neurotransmission through hippocampal CA1 neurons (16).
However, the locus coeruleus is not the only site at which -2 adrenoreceptor agonists produce anesthetizing effects. Like isoflurane, they also suppress nociceptive neurotransmission in the neonatal rat spinal cord, probably by depressing substance P and glutamate-mediated pathways (17,18). Spinal -2 adrenoreceptors may mediate a portion of the antinociceptive effect of medullary and spinal 
(2) opioid receptors (19). Intrathecal or epidural injection of dexmedetomidine in the dog has a powerful antinociceptive effect, one far more potent than that achieved with intracisternal or IV injection (20). In the rat, intrathecal injection of the -2 adrenoreceptor agonist dexmedetomidine produces a dose-dependent inhibition of nociceptive C and innocuous A ß responses of dorsal horn neurons to transcutaneous electrical stimulation (21).
Inhaled anesthetics primarily produce immobility in the face of noxious stimulation by an effect on the spinal cord (22–24). Thus, the observed effects of -2 adrenoreceptor agonists on the spinal cord, and their ability to profoundly decrease the MAC of inhaled anesthetics in an additive manner, suggest the possibility that -2 adrenoreceptors mediate part of the capacity of inhaled anesthetics to produce immobility in the face of noxious stimulation. The present study examined that possibility.
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Methods
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The Committee on Animal Research of the University of California, San Francisco, approved our study of male [Crl:CD(SD)BR] Sprague-Dawley rats weighing 300–350 g obtained from Charles River Laboratories (Hollister, CA). Animals were housed in our animal care facility for at least a week before study under 12 h cycles of light and dark, two per cage, and had continuous access to standard rat chow and tap water before the study.
MAC for isoflurane was measured concurrently in 4–10 rats in each experiment. Each rat was placed in a clear plastic tube through which oxygen containing isoflurane flowed at >0.5 L/min. The tube was capped at each end with rubber stoppers pierced with holes that allowed passage of the oxygen, the entry of a temperature probe, and the exit of the rat’s tail. Rectal temperature was maintained between 36°C and 38.5°C by external heating (infrared lamps) or cooling (application of ice). The isoflurane concentration in the cylinder, monitored with an infrared analyzer (Ohmeda 5250 RGM; Ohmeda, Louisville, CO), was decreased to a concentration (1.0%–1.2%) that permitted movement in response to a 1-min (less if the rat moved) application of a tail clamp with continuous movement of the tail clamp. The concentration was sustained for 30 min before tail-clamp application. After finding that movement occurred, a gas sample was obtained and analyzed using gas chromatography. The isoflurane concentration then was increased by approximately 15%–20% of the preceding value and the process repeated until the rat did not respond to the tail clamp. MAC for the individual rat was estimated as the average of the largest concentration permitting movement and the next largest concentration. The mean and SD for the group of rats were calculated for each study with a particular drug.
We performed several studies of the effect of intraperitoneal injection of -2 adrenoreceptor agonists and/or antagonists on MAC. For each study, we first determined MAC in the absence of injection of agonist or antagonists (control MAC). The concentration of isoflurane then was decreased to that which allowed movement of all animals. The agonist (medetomidine; Pfizer, New York, NY), antagonist (yohimbine; Sigma-Aldrich, St Louis, MO; or atipamezole; Pfizer), or a combination of the two (medetomidine and atipamezole) was injected intraperitoneally, and approximately 30 min later, MAC was re-determined as for the control MAC. Medetomidine is the combination of the active -2 adrenoreceptor agonist dexmedetomidine and the inactive agonist levomedetomidine (4) that is used in veterinary anesthesia.
Paired Student’s t-test were applied, and P < 0.05 was accepted as significant despite the multiple tests applied. We also applied an analysis of variance (ANOVA) with a Dunnett’s test.
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Results
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Yohimbine at 1 mg/kg increased MAC by 11.0% ± 9.6% (mean ± SD; P < 0.02 by a paired t-test). Greater doses either did not change MAC or (as lethal concentrations were approached) decreased MAC (Table 1; Fig. 1). Not shown are the results for the largest two doses, 32 mg/kg and 64 mg/kg. Pairs of rats given these doses died.
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Figure 1. The intraperitoneal administration of 0.8 mg/kg of atipamezole or 1.0 mg/kg of yohimbine increased isoflurane minimum alveolar anesthetic concentration (MAC) by approximately 10%. Larger doses of either drug did not change MAC except that at the largest doses (in the case of yohimbine, doses close to the lethal range), MAC decreased. Asterisks indicate that a paired Student’s t-test showed a difference that was significant at the P < 0.05 level.
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Medetomidine 0.08 mg/kg decreased the MAC of isoflurane by 57% ± 22% (n = 5; P < 0.01 by a paired t-test), and atipamezole 0.4 mg/kg antagonized this effect (3.2% ± 7.2% decrease; n = 4; not significant). This dose of atipamezole given without medetomidine did not increase the MAC of isoflurane (Table 2). Atipamezole 0.8 mg/kg increased MAC by 10.6% ± 10.4% (n = 8; P < 0.05 by a paired t-test). Larger doses either did not change or decreased MAC (Fig. 1).
The ANOVA was significant for both yohimbine (P < 0.001) and atipamezole (P = 0.004). Consistent with the t-tests, the 10% increases in MAC were significant for both yohimbine and atipamezole; the decrease in MAC with the largest dose of yohimbine also was significant.
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Discussion
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We found that blockade of -2 adrenoreceptors with smaller doses of either yohimbine or atipamezole increased the MAC of isoflurane by approximately 10%, whereas larger doses had no effect or decreased MAC (Fig. 1). The smallest dose of atipamezole was sufficient to reverse a substantial anesthetic effect of medetomidine. The doses of yohimbine that decreased MAC approached those that produced death. Our finding that 32 mg/kg and 64 mg/kg of yohimbine produced death confirms the finding by Komulainen and Olson (2) that 20 mg/kg of intraperitoneal yohimbine could be lethal.
Our results confirm and extend those from other studies. Rabin et al. (5) found that depletion of -2 adrenoreceptors by N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline did not change halothane MAC in rats. Bloor and Flacke (6) found that the -adrenoreceptor blocking drug tolazoline at the single dose of 5 mg/kg did not affect halothane MAC in dogs. Two limitations may apply to these previous results. Depletion of -2 adrenoreceptors takes time and allows for the potential development of compensatory pathways, and the use of a single dose of an -adrenoreceptor blocking drug does not allow for the possibility that smaller or larger doses might demonstrate an effect (i.e., does not allow for the possibility that the single antagonist dose used produced incomplete or excessive blockade).
What of the small (about 10%) increase in MAC produced by "therapeutic" doses of atipamezole and yohimbine? These might suggest that a small portion of anesthesia from inhaled anesthetics is mediated by stimulation of -2 adrenoreceptors. Although not tested for isoflurane, nitrous oxide may provide such stimulation (25). An alternative explanation is that a small amount of tonic -2 adrenoreceptor activity exists, and this activity decreases anesthetic requirement. A small increase in MAC results from blockade of this normal activity.
We emphasize that our work indicates that -2 adrenoreceptor agonism does not mediate one aspect of anesthesia produced by inhaled anesthetics, namely the capacity to produce immobility in the face of noxious stimulation. We did not test the possibility that -2 adrenoreceptor agonism might mediate other aspects of anesthesia produced by inhaled anesthetics (e.g., amnesia or the loss of righting reflex). And, of course, although -2 adrenoreceptor agonism does not mediate MAC for inhaled anesthetics, it may play an important role for other anesthetics. Clearly, this is the case for anesthetics such as dexmedetomidine.
In summary, agonism of -adrenoreceptors has a powerful anesthetic effect, one mediated in part by an action on the spinal cord. Agonism of -adrenoreceptors can decrease the MAC of inhaled anesthetics to zero in an apparently additive manner. Nonetheless, because blockade of -adrenoreceptors minimally affects the MAC of isoflurane, we conclude that -adrenoreceptors do not mediate the immobility produced by inhaled anesthetics.
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Acknowledgments
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Supported, in part, by NIH grant 1PO1GM47818–07.
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Footnotes
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Dr. Eger is a paid consultant to Baxter Healthcare Corp. Baxter donated the isoflurane used in these studies.
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References
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Accepted for publication January 29, 2003.
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Inhaled Anesthetics and Immobility: Mechanisms, Mysteries, and Minimum Alveolar Anesthetic Concentration
Anesth. Analg.,
September 1, 2003;
97(3):
718 - 740.
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Abstract
Introduction
