The study hypothesizes that nitrous oxide (N(2)O) releases opioid peptide in the brain stem, which results in inhibition of gamma-aminobutyric acid-mediated (GABAergic) neurons that tonically inhibit the descending noradrenergic inhibitory neurons (DNIN), resulting in activation of DNIN. In the spinal cord, activation of DNIN leads to the release of norepinephrine, which inhibits nociceptive processing through direct activation of alpha2 adrenoceptor and indirect activation of GABAergic neurons through alpha1 adrenoceptor. Arising from this hypothesis, it follows that GABAergic neurons will modulate the antinociceptive effect of N(2)O in diametrically opposite directions at supraspinal and spinal levels. The authors have tested this tenet and further examined the effect of midazolam, a GABA-mimetic agent, on N(2)O-induced antinociceptive effect.
Adult male Fischer rats were administered muscimol (GABA(A) receptor agonist) intracerebroventricularly (icv), gabazine (GABA(A) receptor antagonist) intrathecally (intrathecal), or midazolam intraperitoneally (intraperitoneal). Fifteen minutes later, they were exposed to air or 75% N(2)O and were subjected to the plantar test after 30 min of gas exposure. In some animals administered with midazolam, gas exposure was continued for 90 min, and the brain and spinal cord were examined immunohistochemically.
The N(2)O-induced antinociceptive effect, which was attenuated by icv muscimol, intrathecal gabazine, and intraperitoneal midazolam. Midazolam inhibited N(2)O-induced c-Fos expression (a marker of neuronal activation) in the pontine A7 and spinal cord.
The GABAergic neurons modulate the antinociceptive effect of N(2)O in opposite directions at supraspinal and spinal levels. The pronociceptive effects of enhancement at the supraspinal GABAergic site predominate in response to systemically administered midazolam.
STUDIES over the last several decades have revealed, in part, the underlying mechanisms of nitrous oxide (N2O)-induced analgesic/antinociceptive effect. The initiating action of N2O appears to be the release of opioid peptides in the brain stem, which activates the descending noradrenergic inhibitory neurons, leading to the modulation of nociceptive processing in the spinal cord. 1However, precise neuronal pathways that mediate these effects of N2O are not yet fully understood.
Descending noradrenergic inhibitory neurons are important components of the endogenous pain suppression system and it is thought that they are tonically inhibited under resting conditions by inhibitory γ-amino butyric acid–mediated (GABAergic) neurons. 2Because activation of opioid receptors by endogenous opioid peptides generally mediates inhibitory neurotransmission, we speculate that it is the GABAergic interneurons in the brain stem that are inhibited by N2O, which leads to disinhibition (activation) of the descending noradrenergic inhibitory neurons (fig. 1). In the spinal cord, activation of the descending noradrenergic inhibitory neurons by N2O leads to the release of norepinephrine in the dorsal horn, 3which in turn modulates nociceptive processing by at least two neuronal pathways (fig. 1). One of these neuronal pathways involves the activation of inhibitory GABAergic neurons through α1 adrenoceptors. 4If this theory is correct, GABAergic neurons modulate the antinociceptive properties of N2O in diametrically opposite direction; activation of GABAergic neurons at the supraspinal level will be pronociceptive, while at the spinal level activation of GABAergic neurons will be antinociceptive.
To test this hypothesis, we examined the effects of muscimol (a GABAAreceptor agonist) at the supraspinal level and gabazine (a GABAAreceptor antagonist) at the spinal level on N2O-induced antinociceptive effects using the plantar test in rats. Because of the diametrically opposite effects, we next tested whether and in which direction a systemically administered GABA-mimetic compound will affect the antinociceptive properties of N2O. Thus, we studied the effects of midazolam (a benzodiazepine receptor agonist exhibiting GABA-mimetic effects at GABAAreceptors) on the N2O-induced antinociceptive effect behaviorally and immunohistochemically.
Materials and Methods
Adult male Fischer rats weighing between 250 and 320 g were used throughout the study (B&K Universal, Grimston Aldbrough, Hull, UK). This strain of rat exhibits strong antinociceptive effect in the presence of N2O. 5All animal procedures were carried out in accordance with the United Kingdom (Scientific Procedures) Act of 1986, and the study protocol was approved by the Home Office of the United Kingdom (London, UK). Rats were provided with access to food and water ad libitum and kept in a 12-h light and dark cycle. All efforts were made to minimize animal suffering and reduce the number of animals used. No animal was used for more than one experiment.
Study I: Effect of Supraspinally Administered GABAAReceptor Agonist on N2O-induced Antinociceptive Effect
Rats were anesthetized with 1.5–2.0% isoflurane/oxygen. After stereotactic apparatus was applied to the head, a hole was trephined overlying the left ventricle. Through the hole, a 22G stainless steel cannula was threaded into the lateral ventricle using the atlas of Paxinos and Watson 6with the following coordinates: with bregma as a reference, 4.0 mm posterior, 1.0 mm lateral, and 1.0 mm ventral to the surface. The guide cannula (a BD-7 stainless steel, 11.0 mm in length) was then inserted 3.0 mm into the lateral ventricle and fixed in position with methylmethacrylate resin. After the surgery, 0.6 ml of 0.5% bupivacaine (10 mg/kg) was injected locally around the incision. A minimum period of 4 days elapsed between cannula implantation and experiment being performed. At the conclusion of the experiment, the site of cannulation was examined by injecting pontamine blue dye, of which location within the cerebroventricular was confirmed by autopsy; success rate was 100%.
Drug Administration and Gas Exposure.
A 100-μl Hamilton microsyringe was attached to the chronically implanted cannula by PE10 tubing. The microsyringe was placed on the infusion pump (Pump 22, Harvard Apparatus, South Natick, MA), which was set at a constant flow rate of 5 μl/min for a period of 2 min for drug delivery. Rats were administered 10 μl of either 0.9% sodium chloride (saline) or muscimol (a GABAAreceptor agonist; 3 μg/10 μl). Dosage of muscimol (3 μg) and injection volume (10 μl) were chosen from the data in a study by Hough et al. 7Fifteen minutes after completing drug administration, gas exposure was performed in an exposure chamber (18 inches long, 9 inches wide, and 8 inches high) made of plastic. The chamber and holding area of rats were maintained between 23 and 24°C. Humidity within the chamber was between 35 and 45%. Either air or 75% N2O/25% O2at a flow rate of 4 l/min was continuously delivered into the exposure chamber from an anesthetic machine via an inflow port and exhausted via an outflow port flowing through the Anesthetic Gas Scavenging System (Boyle 2000, Ohmeda, Hatfield, Herz, UK). Gas concentrations, including those for N2O, O2, and CO2, in the chamber were measured continuously by infrared gas spectrometry (Ohmeda 5250 RGM, Ohmeda). Wash-in of the gas mixture to achieve a steady-state concentration within the chamber was achieved in 30 min, after which animals were placed into the chamber through the side door; this procedure did not significantly change the gas concentrations within the exposure chamber. Initially, paper towels were laid down on the floor of the chamber to soak up excrement produced by animals. The paper towels and all excrement collected were removed from the chamber before the plantar test was started. The side door was also used to clean the chamber throughout experimentation, because urine and feces may affect measurements obtained by the plantar test. After 30 min of gas exposure, which coincides with the peak antinociceptive effect of N2O by the tail flick test, 5animals were subjected to the plantar test.
The plantar test, i.e. , thermal nociceptive testing, was performed using the plantar test device (Plantar test 7370, Ugo Basile, Comerio, Italy). The assessor was blinded to drug or vehicle administration. Radiant heat was applied on the plantar surface of hind paws through the floor of the exposure chamber and the paw withdrawal latency (PWL), defined as the time between the activation of the heat source and hind paw withdrawal, was automatically recorded. Approximately 30–60 min before gas exposure, heat intensity was adjusted such that the baseline PWL was approximately 4 s. To avoid tissue damage, a predetermined cut-off time of 10 sec was imposed. Each PWL data consisted of a mean of three trials for each animal. From the PWL, the percentage of maximal possible effect (%MPE) was calculated.
Study II: Effect of Spinally Administered GABAAReceptor Antagonist on N2O-induced Antinociceptive Effect
Intrathecal (Intrathecal) Injection, Gas Exposure, and Plantar Test.
The rat was anesthetized with 1.5% isoflurane/33% O2/66% N2O during the following procedures, which lasted approximately 2 min. Immediately after induction of anesthesia, the rat was shaved around the lumbar area to facilitate identification of the midline of the spine. The animal was then braced over a 50-ml centrifuge tube to splay the intervertebral spaces of the lumbar (L) spine. While firmly holding the rat's vertebral column in one hand, a 27G needle attached to a 50-μl Hamilton syringe was inserted into the intervertebral space between L5 and L6 as described by Hylden and Wilcox. 8Entry into the subarachnoid space was accompanied by a sudden lateral movement of the tail as described by Mestre et al. 9A total volume of 10 μl vehicle with saline (as a control) or vehicle containing gabazine (a GABAAreceptor antagonist; 1 μg/10 μl) was injected, and the needle was removed. The vehicle contained a mixture of 2% pontamine (blue dye)/6% dextrose which was slightly hyperbaric with respect to the cerebrospinal fluid. A dosage of gabazine, 1 μg/10 μl, was chosen because, in preliminary experiments, this proved to be the lowest efficacious blocking dose at the receptor, which does not exhibit its own behavioral effect. (At higher dosages of gabazine administration, animals showed signs of toxicity, e.g. , prominent repetitive tail flexing.) As soon as these procedures were completed, anesthesia was discontinued and the rat awakened within 10 min. Fifteen minutes following intrathecal injection, the rat was exposed to air or 75% N2O/25% O2and subjected to the plantar test 30 min later as described in Study I. After experimental procedures had been completed, the site of injection was confirmed by autopsy. Successful intrathecal administration (13 of 17, 76.5%) was evidenced by the presence of the blue dye within the subarachnoid space; data from unsuccessful injections (4 of 17) were not further considered.
Study III: Effect of Systemically Administered Midazolam on N2O-induced Antinociceptive Effects
Intraperitoneal (Intraperitoneal) Drug Administration, Gas Exposure, and Plantar Test.
Rats were injected intraperitoneally with either saline or midazolam (a benzodiazepine receptor agonist; 1, 5, and 10 mg/kg). These doses are less than those that produce a hypnotic effect that can confound interpretation of the plantar test. Fifteen minutes after injection, rats were exposed to air or 75% N2O/25% O2for 30 min and were subjected to the plantar test as described in Study I.
Immunohistochemical Examination of Spinal Cord and Brain.
In some animals, immunohistochemical studies were performed instead of subjecting animals to nociceptive testing. Rats were given midazolam (1, 5, and 10 mg/kg) intraperitoneally. Fifteen minutes later they were subjected to 90 min of gas exposure to air or 75% N2O/25% O2, which was the time to the peak effect of N2O exposure on c-Fos induction in the spinal cord. 4Animals were then injected with sodium pentobarbital (100 mg/kg) intraperitoneally for perfusion. A thoracotomy was performed by transverse incision at the level of the diaphragm and midline sternotomy. The animals were then perfused with 0.1 m phosphate-buffered saline (PBS) followed by 4% paraformaldehyde in 0.1 m phosphate buffer (PFA) via a 16G cannula inserted through the left ventricle into the ascending aorta. An incision was made in the right ventricle for drainage. Following decapitation, the brain was extracted and the spinal cord was expelled by rapid injection of PBS into the spinal canal at the sacral level. Collected tissues were stored in PFA overnight then transferred into 30% sucrose in PBS for at least 48 h at 4°C. A 5-mm portion of the spinal cord at the lumbar enlargement was cut by a razor blade and was freeze-mounted in embedding matrix. Transverse sections of 30 μm were cut at −15°C, which were collected in PBS; every third section was collected (approximately 40–50 sections per sample). A 2-mm portion of the brain, containing the A7 nuclei region, was cut by a razor blade using specifications from the atlas of Paxinos and Watson 6and freeze-mounted in embedding matrix. Transverse sections of 30 μm were cut at −15°C and were collected in PBS.
Fluorescent Double Staining of c-Fos and Dopamine β-Hydroxylase and Colocalization Quantification.
Free-floating brain sections were first incubated for 1 h in blocking solution consisting of 3% donkey serum (Chemicon, Temecula, CA) in 0.3% Triton X in PBS (PBT). They were then incubated overnight at 4°C on an orbital shaker at 75 r.p.m. with goat antic-Fos antibody (1:2000, Cat. No. sc-52-G, Santa Cruz Biotechnology, Santa Cruz, CA) and rabbit anti-dopamine β-hydroxylase (anti-DBH) antibody (1:2000, Cat. No. DZ1020, Affiniti Research Products, Mamhed, UK) in 1% donkey serum in PBT. The use of c-Fos has been validated as a biochemical marker of neuronal activation in brainstem catecholaminergic neurons using various stimuli. 10–12DBH is a norepinephrine synthesizing enzyme, which is commonly used as an immunohistochemical marker of noradrenergic neurons. Sections were rinsed with PBT, incubated for 1 h in darkness with a mixture of Cy3-conjugated donkey antigoat secondary antibody (1:200, Cat. No. 705–166–147, Jackson Immuno Research Laboratories, West Grove, PA) and fluorescein isothiocyanate (FITC)-conjugated donkey antirabbit secondary antibody (1:200, cat No. AP182F, Chemicon) in 1% donkey serum in PBT, then rinsed with PBS, floated in water, and mounted on glass slides. After drying, the slides were fitted with cover slips with one drop of VectaShield (Vector Laboratories, Burlingame, CA) and were mounted. The four well-preserved undamaged sections were randomly selected from each animal and examined under a fluorescent microscope (Leica DMR microscope, Leica, Wetzlar, Germany). Red staining of the nucleus identified the c-Fos positive neurons. Green staining of the cytoplasm identified the DBH-positive neurons. The A7 region, located according to a rat brain atlas, 6was easily identified by DBH staining of the cytoplasm. Results from four animals for each group were collated and the prevalence of c-Fos/DBH colocalization was calculated. The investigator performing the c-Fos and DBH counting was blinded to the origin of the sections.
Diaminobenzidine Staining and c-Fos Quantification.
Free-floating spinal cord sections were first incubated at room temperature for 30 min in 0.3% hydrogen peroxide in 70% methanol/PBS. Then they were incubated for 1 h in blocking solution consisting of 3% rabbit serum in PBT, followed by overnight incubation with goat anti–c-Fos antibody (1:10000, Cat. No. sc-52-G, Santa Cruz Biotechnology) in blocking solution (1% normal rabbit serum in PBT) at 4°C on an orbital shaker at 75 rpm. The use of c-Fos has been validated as a marker of neuronal activation in the spinal cord. 13,14Sections were then rinsed with PBT, incubated for 1 h with biotinylated rabbit antigoat immunoglobulin (1:200, Vector Laboratories) in 1% normal rabbit serum, rinsed with PBT, and incubated for 1 h with avidin-biotin-peroxidase complex (Vector Laboratories) in PBT at room temperature on an orbital shaker at 75 rpm. Visualization of the immunohistochemical reaction was achieved by incubation with diaminobenzidine with nickel-ammonium sulfate (DAB kit, Vector Laboratories). After the staining procedure was completed, the sections were rinsed in PBS and then rinsed in distilled water, mounted on glass slides (which were dehydrated in 100% ethanol), cleared in 100% xylene, and fitted with cover slips. By diaminobenzidine staining with nickel enhancement, c-Fos positive cells were identified by dense black nuclear staining under a bright field microscope (Olympus Model BX50 Research Photomicroscope, Olympus Optical, Southall, Middlesex, UK). Randomly selected undamaged sections were photographed using a digital camera (Olympus Digital Camera Model C2020Z, Olympus Optical). The number of c-Fos positive cells was counted for each area of the spinal cord, i.e. , laminae I-II (superficial area), laminae III-IV (nucleus proprius area), laminae V-VI (neck area), and laminae VII-X (ventral area), according to the method by Presley et al. 15Five photographs were taken from each rat and the mean number of c-Fos positive cells per section was calculated. Each group was composed of at least four animals, and the number of c-Fos positive cells in each group was calculated as mean ± SD. The investigator was blinded to the treatment group during the counting process.
Data of the plantar test results were expressed as %MPE. Comparisons at the supraspinal and spinal levels were conducted in the following groups: saline/air, drug/air, saline/75% N2O, and drug/75% N2O. Data were analyzed by one-way analysis of variance and Bonferroni correction as a post hoc test. A P value less than 0.05 was considered to be statistically significant.
Results from c-Fos and DBH double staining in the A7 nuclei were compared in the following groups: saline/air, drug/air, saline/75% N2O and drug/75% N2O. Data were analyzed using a contingency table, and Fisher exact test was used as post hoc test. Results from c-Fos single-staining were compared in each laminal scheme in the following groups: saline/air, drug/air, saline/75% N2O, and drug/75% N2O. Data were analyzed by one-way analysis of variance and Bonferroni correction as a post hoc test. A P value less than 0.05 was considered to be statistically significant.
Rats exposed to an air environment were awake and active during the experiment, while those exposed to a N2O environment were excited for the first 5–10 min of exposure, followed by a relatively calm state. No animal exhibited motor dysfunction, as assessed by loss of righting reflex, after administration of agonists or antagonists by any of the three routes of administration.
Study I: Effect of Supraspinally Administered GABAAReceptor Agonist on N2O-induced Antinociceptive Effect
Results from the plantar test are shown in table 1. N2O significantly increased the PWL from a %MPE of 3.6 ± 4.3% (mean ± SD, n = 6) with air to 30.6 ± 7.3% (n = 7). Muscimol had no effect on the %MPE with air (1.2 ± 6.2%, n = 6) but completely attenuated the N2O-induced enhancement of PWL (%MPE of 3.6 ± 9.0%, n = 7).
Study II: Effect of Spinally Administered GABAAReceptor Antagonist on N2O-induced Antinociceptive Effect
Results from the plantar test are shown in table 2. N2O significantly increased the PWL from a %MPE of 3.5 ± 3.4% (mean ± SD, n = 4) to 38.3 ± 10.5% (n = 4). Gabazine had no effect on the %MPE with air (0.9 ± 4.8, n = 4) but significantly lowered the %MPE (15.0 ± 3.7%, n = 4) when compared with the saline/N2O group.
Study III: Effect of Systemically Administered Midazolam on N2O-induced Antinociceptive Effect
Results from the plantar test are shown in table 3. N2O significantly increased the PWL from a %MPE of 2.6 ± 1.3% (mean ± SD, n = 6) to 38.2 ± 6.9% (n = 6). The midazolam/air group showed no difference in %MPE at 1, 5, and 10 mg/kg (n = 6 each) when compared with the saline/air group. Midazolam significantly lowered antinociceptive effect of N2O in a dose-dependent fashion, showing a significant difference in the %MPE at 5 mg/kg (15.5 ± 5.4%, n = 6) and 10 mg/kg (5.8 ± 7.0%, n = 6); no difference at 1 mg/kg (37.8 ± 7.8%, n = 6).
Immunostaining in the A7.
A total of four animals were examined in each group and the results are shown in table 4. In saline/air and midazolam/air groups, the prevalence of c-Fos/DBH colocalization was very low (1.0 and 4.8%, respectively). The saline/N2O group showed significantly higher incidence of colocalization (53.1%); this effect was significantly attenuated by midazolam pretreatment (7.3%).
Immunostaining in the Spinal Cord.
A total of four animals were examined in each group and the results are shown in figure 2. The number of c-Fos positive cells in the entire gray area of the spinal cord section in the saline/air group was 76.2 ± 3.5 (mean ± SD). Exposure to 75% N2O increased the number of c-Fos positive cells to 121.8 ± 11.5. The increase in c-Fos positive cells was significant in laminae III-IV (63.0 ± 4.9 vs. 32.8 ± 1.5, saline/air group) and laminae VII-X (22.0 ± 4.1 vs. 16.0 ± 2.9, saline/air group). The midazolam (10 mg/kg)/air group showed no difference when compared with the saline/air group. In the midazolam/N2O group, c-Fos expression was attenuated when compared with the saline/N2O group; at lower concentration of midazolam (1 mg/kg), a partial reduction occurred while at higher concentrations (5 and 10 mg/kg), almost complete suppression of N2O-induced c-Fos expression was noted.
Descending noradrenergic inhibitory neurons are tonically inhibited under the resting conditions, although the precise neuronal pathways involved have yet to be clarified. 2Evidence to date indicate that N2O induces opioid peptide release in the periaqueductal gray area of the midbrain. We propose that the released opioid peptide inhibits GABAergic interneurons, thereby disinhibiting the descending noradrenergic inhibitory neurons, resulting in modulation of nociceptive processing in the spinal cord (fig. 1). 1Our hypothesis is supported by the finding that the antinociceptive properties of N2O are mitigated when GABAergic inhibition is restored by supraspinal administration of a GABAAreceptor agonist, muscimol (table 1). In the spinal cord, activation of descending noradrenergic inhibitory neurons by N2O leads to the release of norepinephrine, 3which modulates nociceptive processing by at least two different neuronal pathways (fig. 1). One mechanism involves norepinephrine-induced activation of α2 adrenoceptors, 16,17which is thought to decrease neurotransmission in the primary afferent neuron by presynaptic inhibition of calcitonin gene-related peptide and substance P release 18as well as inhibiting firing of the ascending second order neurons, 19thereby interrupting the transmission of nociceptive impulses. Recently, we provided evidence for a second mechanism that involves activation of inhibitory GABAergic neurons 4through α1 adrenoceptors. 20Now we show that spinally administered gabazine, a GABAAreceptor antagonist, significantly attenuates the antinociceptive effect of N2O (table 2), thereby establishing the mechanistic role that activation of spinal GABAergic interneurons plays in transducing the antinociceptive effect of N2O at the spinal level.
Taken together, activation of GABAergic interneurons differentially modulates the antinociceptive effect of N2O depending on whether interneurons are located supraspinally (where they inhibit) or spinally (where they facilitate). In clinical anesthetic practice, drugs that either directly activate or enhance GABA-mediated chloride influx GABAAreceptor are commonly used; therefore, we next inquired whether a systemically administered GABA-mimetic agent would increase (through a spinal site) or decrease (through a supraspinal site) the antinociceptive effect of N2O. As a model GABA-mimetic compound, we used midazolam, which is commonly used as a sedative agent during the perioperative period. Midazolam does not directly “gate” the GABAAreceptor; rather, midazolam exhibits GABA-mimetic properties by enhancing the effect of endogenous GABA within the synapse. 21Because midazolam's sedative and muscle relaxant properties may confound interpretation of behavioral tests, we first established that, at the midazolam doses used (i.e. , 1, 5, 10 mg/kg), there was neither a detectable reduction in motor response nor loss of righting reflex. At these doses, midazolam attenuated N2O-induced antinociceptive effects in a dose-dependent fashion (table 3).
These data suggested that, when administered systemically, GABA-mimetic agents are more likely to enhance tonic GABAergic inhibition of the descending noradrenergic neurons and thus prevent the spinal modulatory effects of N2O. To test this further, we examined the effect of intraperitoneal midazolam on N2O-induced neuronal activation of the pontine A7 nuclei. The N2O-induced activation of the noradrenergic neurons in the A7 (as reflected by increased c-Fos expression in DBH positive cells) was inhibited by systemically administered midazolam (table 4). While N2O activates all three major noradrenergic nuclei in the brain stem (i.e. , A5, locus caeruleus, and A7), 17we have focused our attention on the A7 because only neurons originating from this nucleus terminates on relevant pain processing neurons in the spinal cord. 22–27In experimental settings, disruption of the descending noradrenergic pathway blocks the spinal modulatory and antinociceptive effect of N2O 3,17; our finding that midazolam prevents N2O-induced activation of descending noradrenergic pathway (as evidenced by c-Fos expression in the A7) and thereby its antinociceptive effect corroborates our earlier studies (fig. 2). 3,17
Our data provide a plausible mechanism whereby midazolam blocks N2O-induced activation of descending noradrenergic pathways. At the supraspinal level, midazolam's GABA-mimetic action is expressed because of the presence of endogenously released GABA 2; at this site, it has the effect of tonically inhibiting the descending noradrenergic neurons, thereby mitigating the actions of N2O (table 4). No norepinephrine is released at spinal cord terminals of the descending noradrenergic neurons; therefore, α1 adrenoceptors are not activated and hence GABA interneurons remain quiescent. In the absence of endogenous GABA, midazolam cannot exert its GABA-mimetic antinociceptive action at the spinal cord level. Earlier, Dirig and Yaksh 28reported that intrathecal administration of muscimol (a direct activator of the GABAAreceptor), but not midazolam (an enhancer of activation of GABAAreceptor activation by endogenous GABA), exhibited antinociceptive effect in rats. These data are in agreement with our interpretation. In a series of studies with the combination of midazolam and morphine, Luger et al. also showed that it depended on the site of administration whether midazolam inhibited (icv administration) or potentiated (intrathecal administration) the analgesic of morphine in rats. 29,30
In clinical practice, midazolam is frequently used to sedate patients in preparation for surgery in which N2O and/or opioids are used as part of the anesthetic regimen. If our results can be extrapolated to the clinical settings, then it could mean that the analgesic effects of N2O and opioids may be attenuated by midazolam (and possibly by other indirectly acting GABA-mimetic agents, including barbiturates and propofol). Because volatile anesthetics exhibit both direct and indirect GABA-mimetic properties, 31their effect on N2O and opioid-induced analgesic may be difficult to predict. However, Goto et al. reported that halothane and isoflurane inhibited N2O-induced antinociceptive effect in rats. 32,33More recently, Janieszewski et al. reported that sevoflurane attenuated the analgesic effect of N2O in humans. 34While barbiturates and propofol alone have been demonstrated to have a hyperalgesic action, 35–38we did not observe a hyperalgesic effect of icv muscimol (3 μg). Apart from the fact that these compounds differ in their ability to directly gate chloride conductance through the GABAAreceptor, it is possible that our experimental paradigm is less stressful than that used in the other reports. 35–38Thus the basal state in those reports may in fact be one of stress-induced analgesia and the administration of propofol, barbiturates, and midazolam may interrupt descending noradrenergic inhibitory pathways and reduce the latency to withdrawal from the noxious stimulus, providing the interpretation that the drugs are hyperalgesic. 35–38We posit that, under our experimental conditions, the basal state is stress-free because the subject is unrestrained; in this basal state, the descending noradrenergic inhibitory neurons are tonically inhibited by GABAergic neurons at the supraspinal level. Under these circumstances, the addition of an exogenous GABA agonist, such as muscimol, does not contribute further to what is already being effected by endogenous GABA.
In summary, we have demonstrated that GABAergic neurons differentially modulate the antinociceptive effects of N2O at supraspinal and spinal levels in Fischer rats. Systemic administration of midazolam, the GABA-mimetic agent, attenuates the N2O-induced antinociceptive effects, assessed both behaviorally and immunohistochemically. If these findings are extrapolated to humans, it suggests that the analgesic effect of N2O may be attenuated by midazolam.