| JOURNAL HOME | CME HOME | THIS MONTH | PAST ISSUES | ETOC | COLLECTIONS |
| AUTHORS | REVIEWERS | EDITORIAL BOARD | FEEDBACK | RSS | HELP |
| ||||||||||||||
| ||||||||||||||
|
|
|
|
||||||||||||
|
||||||||||||||
From the Departments of *Anesthesiology, Clinical Neurosciences, and Pharmacology and Therapeutics, Faculty of Medicine, University of Calgary, Calgary, Canada; and Preclinical CNS Research, UCB S.A., Braine-l’Alleud, Belgium.
Address correspondence to D. Archer, MD, MSc, Department of Anesthesiology, Foothills Medical Centre, 1403 29th St., NW, Calgary, Alberta, Canada T2N 2T9. Address e-mail to david.archer{at}calgaryhealthregion.ca. Reprints will not be available from the authors.
Abstract
BACKGROUND: As part of an increase in excitability, small doses of pentobarbital, propofol, and midazolam induce an increased sensitivity to pain. Specific therapy to prevent or reduce this excitability may offer advantages over current clinical management with analgesics and sedatives. The pharmacological profile of the novel antiepileptic drug, levetiracetam, suggests that it may reduce the intensity of the excitatory stages of anesthesia.
METHODS: We examined the influence of levetiracetam on the reduction of the nociceptive reflex threshold in rats by sedative doses of pentobarbital, propofol, and midazolam. Measurements of nociceptive reflex threshold to pressure and heat were made and then repeated after intraperitoneal injection of saline or one of three doses of levetiracetam (100, 200, 500 mg/kg). Pentobarbital (30 mg/kg), propofol (30 mg/kg), or midazolam (1.9 mg/kg) were then administered. The reflex threshold was measured every 10 min, starting at 5 min after the sedative injection, until 65 min had elapsed.
RESULTS: Levetiracetam did not alter nociceptive reflex threshold in nonsedated animals (P = 0.11) or influence the degree or duration of sedation. The three anesthetic/sedative drugs reduced the nociceptive reflex threshold by 20%–30% of control values. Levetiracetam reduced the hyperreflexia associated with pentobarbital and midazolam (P < 0.05), but not propofol.
CONCLUSIONS: These findings support further investigation into the role of levetiracetam in the prevention of anesthetic-induced excitability.
The purpose of the present study was to test the hypothesis that pretreatment with the antiepileptic drug, levetiracetam can prevent the drug-induced hyperalgesia induced by small doses of anesthetics. Levetiracetam (2R)-2-(2-oxopyrrolidin-1-yl) butanamide, KEPPRA®), a novel pyrrolidone derivative, is approved for adjunctive treatment of refractory partial seizures in adults in Europe and North America. Although the precise mechanism of action of levetiracetam is not fully understood, the drug binds specifically to synaptic vesicle protein 2A in both brain (1) and spinal cord (2). The latter finding suggests that the drug may act presynaptically, modifying neurotransmitter release. Among its pharmacological properties is a distinctive desynchronizing effect on epileptic activity documented in rat hippocampal slices (3), suggesting that levetiracetam may blunt collective neuronal responses such as those seen during the excitatory phase of anesthesia (4). Levetiracetam has been reported (5) to be devoid of analgesic properties and does not alter the electrophysiology of normal neurons.
The present study examined the contributions of two additional factors to anesthetic-induced decreases in nociceptive reflex threshold: the modality of noxious stimulation and the choice of anesthetic/sedative. Studies (6) have revealed specific receptors for noxious heat, distinct from receptors that respond to noxious pressure. Because nociceptor receptor activity is susceptible to modulation by both cytokines and drugs (6), we speculated that there may be differences in drug-induced hyperalgesia that are modality- and anesthetic-dependent. Consequently, in the present study, we have examined the effects of small doses of pentobarbital, midazolam, and propofol with both heat and pressure as noxious stimulation.
The primary goal of the study was to test the hypothesis that premedication with levetiracetam would block the decrease in nociceptive reflex threshold that is induced by sedative doses of pentobarbital, midazolam, and propofol. In addition, we sought evidence that premedication with levetiracetam produced sedative effects, either on its own or in combination with the anesthetics/sedatives.
METHODS
Male Sprague-Dawley rats (Charles River Canada, QC, Canada) initially weighing 300–350 g, were housed in groups of two or three, with free access to food and water and exposed to a 12-h light–dark cycle for at least 1 wk before testing. Study protocols, approved by the Faculty of Medicine Animal Care Committee (University of Calgary, Calgary, Alberta, Canada), were designed to comply with the guidelines of the International Association for the Study of Pain and the Canadian Council for Animal Care. Each animal was subjected to a maximum of three trials, with at least 1 wk rest between each trial.
The study was designed as a three-factorial experiment examining the effects of levetiracetam, thermal or pressure noxious stimulation, and type of anesthetic on anesthetic-induced reduction of nociceptive reflex threshold. The secondary outcome measure was sedation. Power analysis was performed using variance estimates from previous studies in our laboratory. A group size of six to eight was predicted to have a likelihood of >80% for detecting a 50% reduction in postanesthetic hyperalgesia.
Nociceptive reflex threshold was determined by paw withdrawal latency to radiant heat and the pressure threshold for the tail flick response. The secondary outcome measure was sedation, quantified with a sedation score and the time to initial recovery, as outlined below.
Response to acute noxious heat was evaluated with the latency for withdrawal of the hindpaw using a custom-made Hargreaves box as previously described (7). The testing chamber is a box (19 x 28 x 29 cm) with acrylic walls and a glass floor. The floor is heated by a lamp bulb (Radius tungsten halogen lamp, model EJY, 19 V, 80 W; General Electric, Glen Allen, VA) that projects through an aperture (5 x 10 mm) in the cover of a housing installed below the glass floor. The circuit of the device consists of a switch, a timer, a photocell beneath the housing aimed at the aperture and the lamp. Closure of the switch activated the lamp and the timer. Movement of the hindpaw from over the aperture extinguished the light reflected on the photocell, opening the circuit, stopping the timer and the lamp. The device can measure times with a sensitivity of 0.1 s. Withdrawal latency in untreated rats ranged from 9–12 s, with a cutoff of 18 s.
The response to acute noxious mechanical stimulation was evaluated with the tail flick pressure threshold (8). The first motor response to pressure applied to the tail was measured with an Analgesy-Meter (Stoelting Instruments, Chicago). The device applies increasing pressure by displacing a weight along a lever that is resting on the tail. Measurements of tail pressure were recorded in centimeters of displacement along the lever and then converted with standard tables to grams of mass. Each animal was allowed sufficient time, usually 10–15 min, to acclimatize to confinement in an acrylic experiment box (Stoelting Instruments). A constantly increasing pressure was applied to randomly selected points within the distal 2 cm of the tail supported on a plinth. The end-point was defined as the first movement of the tail that could displace it from the plinth.
Sedation was quantified with a sedation score determined by the response to light touch with a fingertip to the rat’s whiskers (0 = awake, fully responsive to surroundings, 1 = not responsive to surroundings, rapid response to whisker stimulation, 2 = slow response, 3 = unresponsive to whisker stimulation). The righting reflex was defined to be present if the rat made any effort to reassume the prone position within 1 min after being placed supine. Sedation score and righting reflex were measured immediately after the measurement of the nociceptive reflex threshold.
Experiments were performed in a separate quiet room by a single observer blinded to the nature of the leviracetam treatment. The nociceptive reflex threshold to heat and pressure was measured in separate experiments on different days. The time course of each experiment was as follows: 1) Animals acclimatized for 10–15 min to the testing conditions after which the first control (Control 1, C1) measurements were made. 2) Levetiracetam (100, 200, and 500 mg/kg) or drug vehicle (saline) was injected. Fifteen minutes later, a time which has been shown to be adequate for absorption of levetiracetam in rats (9), the second control measurements were made (Control 2, C2). 3) Immediately after C2 the anesthetic/sedative was injected. Measurements were performed 5, 15, 25, 35, 45, 55, and 65 min after the injection of anesthetic or sedative. At each measurement time, the reflex threshold was measured, a sedation score was assigned and the presence or absence of the righting reflex was determined in the order described.
Selection of the treatments within each anesthetic drug group was done by randomized block design. Drug vehicle control injections were performed with saline; these control subjects were included within each of the blocks studied. All drugs were administered by intraperitoneal injection (i.p.). Pentobarbital (30 mg/kg), midazolam (1.9 mg/kg), or propofol (30 mg/kg) were then administered i.p. based upon the results from a previous study (7). Rats that were injected with saline/saline underwent the same testing sequence described above, served as time controls.
Results are presented as mean ± se of the mean (sem). Nociceptive reflexes were quantified by paw withdrawal latency and tail flick pressure threshold, measured in seconds and grams, respectively. For each trial, the values obtained at 5, 15, 25, 35, 45, and 65 min were expressed as a percentage of the C2 value (after levetiracetam, preanesthetic control), and the area between the baseline and nociceptive threshold curve (AUC) was calculated using the trapezoid rule. The values for AUC were used as the primary outcome variable three-factor analysis of variance (ANOVA) (stimulus, anesthetic/hypnotic, and levetiracetam dose). Individual subgroups were analyzed by multiple pairwise comparisons using the Holm–Sidak method with an overall significance level set at 0.05. Statistical analysis and preparation of graphs were performed using SigmaStat® and Sigmaplot® software (SPSS, Chicago), respectively. The influence of levetiracetam on nociceptive reflex threshold was analyzed separately by paired t-test of the values of paw withdrawal latency or tail pressure threshold before (C1) and after (C2) the injection of levetiracetam (0, 100,200, 500 mg/kg).
Sedation was characterized by two outcome variables: the maximum sedation score achieved after injection (maximum sedation score) and the first time that a decrease of 1 U of sedation scores ("initial recovery time") were observed. For the analysis of sedation, the values for the paw withdrawal latency and tail flick groups were pooled. As the results were not normally distributed, the findings were analyzed by one-way ANOVA on ranks (Kruskal–Wallis test). To compare the sedative effects of the selected doses of pentobarbital, midazolam, and propofol, maximum sedation score and recovery time were compared in animals that did not receive levetiracetam. To examine the sedative effects of levetiracetam, maximum sedation scores and recovery times within each of the anesthetic/sedative groups were compared according to the dose of levetiracetam.
RESULTS
A total of 182 experiments were performed on 78 animals. One study was excluded because agitation precluded nociceptive reflex measurement. Pentobarbital, propofol, and midazolam reduced the latency for a response to noxious heat and the tail pressure stimulus required to evoke a response (Fig. 1). This effect was evident within 5 min of drug injection and persisted throughout the 65 min of the study (Fig. 1). When measured 15 min after injection, levetiracetam did not alter nociceptive reflex threshold (Table 1). The modality of the noxious stimulus, the anesthetic/hypnotic administered, and the levetiracetam dose all had a significant independent effect on the nociceptive reflex threshold (Table 2, Fig. 2). Three-factor ANOVA showed that the area under the nociceptive threshold–time curve (AUC) using the heat stimulus was greater than that using pressure (P < 0.0001) and was greater with pentobarbital than with propofol, which was similar to midazolam (P < 0.0001). The subgroup analysis (Fig. 2) showed that levetiracetam decreased the hyperalgesia caused by pentobarbital and midazolam, but not that caused by propofol.
|
|
|
|
The results for the measures of sedation in the study are summarized in Figure 3. In the animals that did not receive levetiracetam, the maximum sedation score achieved with pentobarbital was more than that seen with either propofol or midazolam, whereas the "initial recovery time" for propofol was shorter than that for pentobarbital and midazolam. Within each anesthetic/sedative group, levetiracetam treatment did not have an effect on the measures of sedation used in the present study (Fig. 3).
|
In both saline- and levetiracetam-treated animals, the righting reflex was consistently lost when the sedation score achieved a value of 3, and was regained with a sedation score of 2. Because the results for righting reflex correlated perfectly with the sedation scores, no further analysis was performed.
DISCUSSION
The present results support the hypothesis that pretreatment with levetiracetam can blunt the reduction of nociceptive reflex threshold that is induced by small doses of anesthetics. The study confirmed that the modality of stimulation and the choice of anesthetic/sedative can contribute significantly to drug-induced hyperalgesia. In addition, the presence of a significant interaction between levetiracetam dose and choice of anesthetic (P = 0.048, Table 2) suggests that the antihyperalgesic effect of levetiracetam is drug-dependent. The magnitude of the reduction of hyperalgesia by levetiracetam is similar to that seen with intrathecal acetazolamide (7).
Time-related effects, including repeated presentation of noxious stimulation, if present, were small in comparison with the hyperalgesic effects (Fig. 1). Levetiracetam did not alter nociceptive reflex threshold (Table 1), confirming previous findings that, in contrast with other antiepileptic drugs, such as lamotrigen and gabapentin, levetiracetam does not demonstrate analgesic properties (10). Although the anesthetic/sedatives produced different depth and durations of sedation (Fig. 3), there was no evidence that either of the measures of sedation were significantly influenced by pretreatment with levetiracetam. Based on these findings, we feel that it is very unlikely that the reduction in hyperalgesia by levetiracetam can be explained by enhancement of the anesthetic potency of pentobarbital or midazolam.
Using pressure analgesimetry in human subjects, Dundee et al. (11–13) systematically examined the influence of small concentrations of volatile and IV anesthetics on somatic pain, showing increased responsiveness to painful stimulation after patients had received barbiturates, halothane, and alphathesin. In contrast, propofol showed analgesic properties, in response to noxious pressure (14) and to laser-induced heat (15). In our laboratory, administration of low doses of barbiturates, propofol, and midazolam to rats has consistently induced a transient period of hyperalgesia (8,16). Zhang et al. (17) reported hyperalgesic effects of subanesthetic concentrations of volatile anesthetics in rats.
The clinical relevance of the pronociceptive effects of small concentrations of anesthetics is controversial. We feel that it is unlikely that the drug-induced hyperalgesia by anesthetics contributes to long-lasting sensitization of the central nervous system (CNS), because repeated noxious stimulation during the excitatory stage of anesthesia does not lead to either persistent hyperalgesia or the activation of the immediate-early-gene, c-fos, that is seen with tissue injury (18). Rather, our results (4,19) suggest that the phenomenon of decreased nociceptive reflex threshold by small concentrations of anesthetics (8,16,17) is a measure of the excitatory effects of these drugs in the CNS.
During Guedel Stage II of anesthesia (20), patients are in an excitable state in which they exhibit abnormally brisk responses to external stimulation. The neural mechanisms that are responsible for the excitation are not well understood and it is not known whether the mechanisms responsible are modality-specific or whether there are common underlying mechanisms throughout the CNS. For barbiturates, three lines of evidence support a role for a common mechanism. First, pharmacodynamic modeling supports a common site of action for electroencephalographic (EEG) effects and nociceptive reflex effects. The characteristics of the site of action for nociceptive reflex effects of barbiturates (19) are similar to those of the EEG site of action (21), and activation of hippocampal EEG correlates with nociceptive reflex effects (4). Second, the concentration of barbiturates that enhance synaptic transmission in the hippocampus (22) are similar to those that decrease nociceptive reflex threshold (8,16). Finally, there is evidence that 
-aminobutyric acid-mediated depolarization (23), which plays a role in enhanced neuronal excitability by barbiturates in the hippocampus and cerebral cortex, also participates, in the spinal cord (7), in decreased nociceptive reflex thresholds by these drugs. Taken together, this evidence supports the concept that enhancement of -aminobutyric acid-mediated depolarization by small concentrations of anesthetics may increase neuronal excitability in diverse pathways in the nervous system.
We are not aware of any previous reports of the effects of levetiracetam on emergence excitement or on anesthetic-induced hyperalgesia. Ardid et al. (5) reported that levetiracetam, in doses of 20–540 mg/kg, had an antihyperalgesic effect in neuropathic pain models in rats. The highest dose of levetiracetam tested in the present study (500 mg/kg) had only minimal effects on sedation and did not demonstrate analgesic effects. The levetiracetam dose associated with a 50% incidence of psychomotor impairment in the rotorod test in rodents (toxic dose, TD50) has been reported (24) to be 1060 mg/kg, indicating a wide margin of safety for the doses that were effective in the present study.
In summary, the results of the present study suggest that levetiracetam may be effective in preventing hyperalgesia during emergence from anesthesia with pentobarbital or midazolam. The antihyperalgesic effect was observed with levetiracetam doses of 200 and 500 mg/kg and was observed with both thermal and mechanical stimulation. These results support further investigation of pyrrolidone derivatives for the prevention of excitatory effects of small concentrations of anesthetics.
ACKNOWLEDGMENTS
The authors thank Dr. Roy Massingham for his encouraging and helpful suggestions in the design of the present study, and Mrs. Valerie Repper for editorial assistance in the preparation of the manuscript.
Footnotes
Accepted for publication September 18, 2006.
Supported by UCB.
Presented in part at the annual meeting of the International Anesthesia Research Society, Honolulu, Hawaii, March 2005.
REFERENCES
This article has been cited by other articles:
|
|
 |
|
  D. Archer and S. Roth Anesthetics are exciting! Implications of pharmacologically-induced nociceptive hyperreflexia/Les anesthesiques, c'est excitant... Implications de l'hyperreflectivite nociceptive provoquee par pharmacologie Can J Anesth, January 1, 2008; 55(1): 6 - 10. [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
