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Anesthetic Pharmacology: Research Report

Meloxicam, a Specific COX-2 Inhibitor, Does Not Enhance the Isoflurane Minimum Alveolar Concentration Reduction Produced by Morphine in the Rat

Santos, Martín, DVM, PhD; Kunkar, Viviana, DVM; García-Iturralde, Palma, DVM; Tendillo, Francisco J., DVM, PhD, Dip.ECVA

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doi: 10.1213/01.ANE.0000097849.18820.C4
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Nonsteroidal antiinflamatory drugs (NSAIDs), as well as opioids, have represented a therapeutic basis of analgesic treatment. Clinical studies suggest a synergistic analgesic effect when opioids and NSAIDs are administered in combination (1). The combination of NSAIDs and opioids is more analgesic than the summed effect of each drug given separately (1–3). The mechanism of action of this effect has been identified as a synergistic synaptic interaction between opioids and NSAIDs (4,5). μ-opioid receptors activation in the periaqueductal gray (PAG) causes a presynaptic inhibition of GABA (γ-aminobutyric acid) release that is mediated by activation of a voltage-dependent K+ channel via 12-lipoxygenase metabolites of arachidonic acid (4).

NSAIDs block the action of the enzyme cyclooxygenase (COX) in the process of converting arachidonic acid into mediators of inflammation. COX appears in two isoforms: COX-1, which is observed under physiological conditions and is responsible for the synthesis of prostaglandins that protect the organism, and COX-2, the isoform induced by inflammatory stimuli and pathological conditions. The NSAIDs are classified depending on the activity of each isoform as nonselective COX inhibitors and specific COX-2 inhibitors.

Opioids reduce the minimum alveolar concentration (MAC) of inhaled anesthetics (6), and a synergistic effect of nonselective COX inhibitors on isoflurane MAC (MACISO) reduction produced by morphine has been observed in the rat (7). Because of the development of specific COX-2 inhibitors that increase the analgesic and antiinflammatory effects while the adverse effects are minimized (8,9), the influence of specific COX-2 inhibitors on inhaled anesthetics MAC reduction produced by opioids should be studied.

The present study was designed for the purpose of evaluating the influence of meloxicam on the morphine reduction of isoflurane MAC in the rat.

Methods

Sixty-four female Wistar rats with a mean body weight of 200 ± 10 g (range, 190–210 g) were allowed to acclimatize for at least 1 wk before the experimental procedure. Animals were housed in groups of 8 to 10 in Plexiglas cages with free access to food and water and maintained on a 12 h light/12 h dark cycle (light on at 7 am) under controlled environmental conditions (relative humidity, 50%–70%; temperature, 20°C ± 2°C). All the studies were performed during the morning (9 am–12 pm). All animals were handled according to the guidelines set in the “Guide for the Care and Use of Laboratory Animals” published by the Institutes of Health. The institutional animal care and use committee approved the study.

The unmedicated rats were placed in an induction chamber to which 5% isoflurane in a continuous oxygen flow of 3 L/min was directed. Once the animals were anesthetized, tracheal intubation was performed using a 16-gauge polyethylene catheter with the animal positioned in dorsal recumbency. Then a flexible, blunt-tip, wire guide was inserted into the trachea with an otoscope and used to direct the endotracheal catheter. The catheter was previously marked and advanced with the tip located 3–5 mm cranially to the carina. After the correct position of the catheter was ascertained, it was connected to a small T piece of minimal dead space (<0.2 mL). The proper catheter position of the animals was checked at the end of the study. Fresh gas flow to the T piece was adjusted to 1 L/min, and isoflurane concentration was adjusted as necessary after anesthetic reflex assessment. During the study, the rats were breathing spontaneously.

The carotid artery was catheterized with a 24-gauge polyethylene catheter via surgical cut-down. This access allowed for arterial blood sampling and blood pressure measurement via a calibrated pressure transducer. Arterial blood pressure was recorded continuously. Arterial blood gases were measured occasionally during MAC assessment and then at the end of study period to ensure values were within normal limits for pH (7.30–7.40), pressure of oxygen (Pao2) (>90 mm Hg), and pressure of carbon dioxide (Paco2) (40–45 mm Hg). Rectal temperature was also monitored and maintained at normothermia (between 37°C and 38°C) by means of a total temperature management system. A caudal tail vein was cannulated using a 24-gauge polyethylene catheter for the administration of drugs. Inspired isoflurane concentrations were further decreased to 1.5%, a value close to the average MACISO for rats before the first MACISO determination.

Once this concentration was achieved, one of the solutions being tested in this study was randomly selected for IV administration and the series was divided into eight groups of animals depending on which of these solutions was used. SAL (n = 8) received an IV bolus of 1 mL of saline solution. ASP (n = 7) received an IV bolus of 0.5 mL of saline solution + 30 mg/kg of aspirin diluted in 0.5 mL of saline solution. MOR (n = 8) received an IV bolus of 1 mg/kg of morphine diluted in 0.5 mL of saline solution + 0.5 mL of saline solution. MOR + ASP (n = 7) received an IV bolus of 1 mg/kg of morphine diluted in 0.5 mL of saline solution + 30 mg/kg of aspirin diluted in 0.5 mL of saline solution. MEL 1 mg/kg (n = 9) and MEL 3 mg/kg (n = 9) received an IV bolus of 0.5 mL of saline solution + 1 mg/kg or 3 mg/kg of meloxicam respectively diluted in 0.5 mL of saline solution. MOR + MEL 1 mg/kg (n = 7) and MOR + MEL 3 mg/kg (n = 9) received an IV bolus of 1 mg/kg of morphine diluted in 0.5 mL of saline solution + 1 mg/kg or 3 mg/kg of meloxicam, respectively, diluted in 0.5 mL of saline solution.

All drugs were administered IV in 3–5 min to reduce cardiovascular and respiratory effects when administered more quickly. MACISO was determined 30 min after drug administration.

Intratracheal gas sampling was used to measure anesthetic gas concentration for determination of the MAC. Inspired and end-tidal isoflurane concentrations were obtained continuously from gas drawn from a fine tubing inserted through the endotracheal catheter over a hole in the T piece and with the tip located at the level of the carina. The proximal end of the catheter was connected to a calibrated infrared-absorption analyzer with a 60 mL/min aspiration flow of the gas sample. After every step change in isoflurane concentration delivered by the anesthetic circuit, at least 15 min were allowed for equilibration maintaining a constant alveolar concentration and an alveolar to inspired ratio (FA/FI) more than 0.95.

The MACISO value was established according to the method described by Eger et al. (10). A painful noxious stimulus was applied with an 8-in. hemostat clamped to the first ratchet lock on the tail for 60 s. The tail was always stimulated proximal to a previous test site. A positive response was considered when a gross purposeful movement of the head, extremities, or body, or a combination of these, was observed, whereas a negative response was the lack of movement, swallowing, chewing, or tail flick. The isoflurane concentration was then reduced in decrements of 0.1% until the negative response became positive. The MACISO was defined as the average of the smallest concentration preventing a positive response and the largest concentration allowing a positive response to the supramaximal painful stimulus. For each rat, MAC was determined in duplicate. The person assessing the response was blinded as to the drugs administered at each group.

Statistical analysis of data was performed using the SPSS 10.0 software program (SPSS Inc., Chicago, IL). All data were grouped and summarized as mean ± sd. Analysis of variance was performed and post hoc comparison of the groups was performed using the Tukey test. A P < 0.05 value was considered statistically significant.

Results

The average MACISO value determined in the saline group was 1.56% ± 0.12%. The groups treated with aspirin, 1 mg/kg meloxicam, and 3 mg/kg meloxicam did not express any statistically relevant changes in comparison with the saline group nor among them (aspirin 1.55% ± 0.11%; 1 mg/kg meloxicam 1.56% ± 0.12%; and 3 mg/kg meloxicam 1.53% ± 0.11%). The administration of morphine showed a significant reduction of the MACISO, which attained 1.35% ± 0.08% (13.46% reduction in MACISO); nevertheless, MACISO after administration of morphine + aspirin was 1.19% ± 0.05% (23.71% reduction in MACISO). The administration of morphine + meloxicam 1 mg/kg and morphine + meloxicam 3 mg/kg significantly reduced the MACISO just as in the group where only morphine was administered (morphine + 1 mg/kg meloxicam 1.36% ± 0.04% (12.82% reduction in MACISO) and morphine + 3 mg/kg meloxicam 1.37% ± 0.08% (12.18% reduction in MACISO) (Fig. 1).

Figure 1.
Figure 1.:
Isoflurane minimum alveolar concentration after administration of either saline (SAL) or morphine alone (MOR) or in combination with aspirin (ASP) or meloxicam (MEL) in rats. *Statistically significant (P < 0.05) respect to SAL, ASP, MEL 1 mg/kg and MEL 3 mg/kg groups. #Statistically significant (P < 0.05) respect to the MOR, MOR + MEL 1 mg/kg, and MOR + MEL 3 mg/kg groups.

During the MAC determination there were no differences in heart rate, arterial blood pressure, pH, Pao2, Paco2, and temperature among the studied groups (Table 1). A decrease in arterial blood pressure was observed during the administration of morphine but not during aspirin or meloxicam administration. Morphine, but not aspirin or meloxicam, produced transient bradypnea during its administration.

Table 1
Table 1:
Cardiorespiratory Effect at the Moment of the Determination of Isoflurane Minimum Alveolar Concentration After Administration of Saline or Morphine Alone or in Combination with Aspirin or Meloxicam in Rats

Discussion

The administration of meloxicam does not potentiate the MACISO reduction produced by morphine in the rat. Meloxicam is one of the most recent NSAIDs to be added to the family of specific COX-2 inhibitors that exhibit antiinflammatory, analgesic, and antipyretic properties. Its mean total body clearance in the rat after IV administration of 1.0 mg/kg is very slow (CL, 0.015 L · kg−1 · hour−1) and provides a long-lasting elimination half-life (t1/2 13.4 hours) and mean residence time (MRT, 18.0 hours); these disposition pharmacokinetic variables are similar to values in humans (CL, 0.010 L · kg−1 · hour−1; t1/2 13.7 hours and MRT 18.2 hours) after IV administration of 0.43 mg/kg (total dose of 30 mg for a body weight of 70 kg) (11), which is much larger than the 0.125–0.250 mg/kg/day clinically used.

In a mono-arthritic rat model, intraperitoneal administration of meloxicam (ID50 = 0.4 mgL · kg−1 · day−1) reduced swelling and stiffness of the inflamed joint, joint hyperalgesia and spontaneous pain-related behavior (12). Meloxicam (3 mg/kg) and aspirin (100 mg/kg) showed almost equal antiinflamatory potency against 5 hours carrageenin-induced pleurisy (13). We used 30 mg/kg aspirin, which may be considered in the small dose range in the rat, and two different doses of meloxicam (1.0 mg/kg and 3.0 mg/kg).

NSAIDs exert their analgesic effect not only through peripheral inhibition of prostaglandin synthesis but also through a variety of other peripheral and central mechanisms (14). The combination of NSAIDs and opioids is more analgesic than the summed effect of each drug given separately (1–3). The midbrain region PAG is rich in opioid receptors and endogenous opioids. It is a major target of analgesic action in the central nervous system, and it is a critical brain region for the synergistic analgesic actions between opioids and NSAIDs. μ-opioid receptors activation in the PAG causes a presynaptic inhibition of GABA release that is mediated by activation of a voltage-dependent K+ channel via 12-lipoxygenase metabolites of arachidonic acid. Furthermore, the action of μ-receptor agonists in the PAG is potentiated by inhibitors of COX and 5-lipoxygenase because more arachidonic acid is available for conversion to 12-lipoxygenase products (4). The mechanism of action of this effect has been identified as a synergistic synaptic interaction between opioids and NSAIDs (4,5).

Analgesic doses of opioids clearly reduce the MAC of inhaled anesthetics (6); however, we observed that administration of meloxicam, like other NSAIDs, does not reduce inhaled anesthetic requirements (7,15,16).

Aspirin synergistically potentiates MACISO reduction produced by morphine in the rat (7), a result that is confirmed in the present study. Nevertheless, meloxicam does not potentiate the MACISO reduction produced by morphine administration in rats. Similar results have been observed in dogs after administration of butorphanol in combination with carprofen, another specific COX-2 inhibitor (15). The inhibition of GABAergic neurotransmission in the PAG appears to be a key mechanism for activating descending pathways that inhibit nociception (17). The greater potency of the nonselective COX inhibitors over the specific COX-2 inhibitor in potentiating the presynaptic inhibitory effects of morphine indicates that the synergistic effects of NSAIDs are likely to be mediated by inhibition of COX-1 (18), although both COX-1 and COX-2 are found in the central nervous system (19). These finding suggest that COX-1 selective NSAIDs may have important roles as analgesics in the central nervous system, particularly in combination with opioids (17).

Also, opiates and NSAIDs act at the spinal level, and a significant component of MAC occurs at the spinal cord (20). Some authors report that intrathecal COX inhibitor administration failed to produce any potentiation of morphine antinociception (21). Others report that nonselective inhibitors and COX-2 inhibitors potentiate morphine antinociception at the spinal level (22–23).

The continuous draw of 60 mL/min for end-tidal isoflurane analysis probably is large given the minute volume of a rat. The end-tidal reading probably slightly overestimated the true alveolar concentration; however, this does not matter in reference to the conclusions because control and experimental animals would have the same limitation.

In summary, this study shows a lack of synergistic effect of meloxicam on MACISO reduction produced by morphine administration in the rat. However, further research is necessary to show the effect in humans and study the differences between other NSAIDs.

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