Analgesic interaction between intrathecal midazolam and glutamate receptor antagonists on thermal-induced pain in rats.

BACKGROUND: Two major neurotransmitters, gamma-aminobutyric acid (GABA) and the excitatory amino acid, glutamate, may be involved in nociception in the spinal cord. GABA and glutamate receptors may operate in concert to modify signals in the central nervous system. The purpose of this study was to investigate the spinal analgesic interaction between midazolam, a benzodiazepine-GABA(A) receptor agonist, and two glutamate receptor antagonists on acute thermal nociception. METHODS: Sprague-Dawley rats were implanted with chronic lumbar intrathecal catheters and were tested for their tail withdrawal response by the tail flick test after intrathecal administration of saline, midazolam (1-100 microg), AP-5 (1-30 microg), or YM872 (0.3-30 microg). AP-5 is an N-methyl-D-aspartate (NMDA) receptor antagonist and YM872 is an alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor antagonist. The combination of midazolam and the other two agents were also tested by isobolographic analyses. Motor disturbance and behavioral changes were observed. RESULTS: Dose-dependent increases in the tail flick latency were observed with midazolam, AP-5, and YM872 with 50% effective dose values of 1.57+/-0.34 (SEM) microg, 5.54+/-0.19 microg, and 1.0+/-0.22 microg, respectively. A potent synergy in analgesia with decreased behavioral changes and motor disturbance was obtained when combining midazolam with AP-5 or YM872. CONCLUSIONS: Spinally administered midazolam and an NMDA- or an AMPA-receptor antagonist exhibited potent synergistic analgesia on acute thermal nociception in rats. Side effects, shown by behavioral changes and motor disturbance, decreased with the combination of the agents. These results point out an important direction for the study of acute nociception.     

The analgesic interaction between intrathecal clonidine and glutamate receptor antagonists on thermal and formalin-induced pain in rats

Clonidine, an alpha(2) adrenergic receptor agonist, inhibits glutamate release from the spinal cord. We studied the interaction of intrathecally administered clonidine and glutamate receptor antagonists on acute thermal or formalin induced nociception. Sprague-Dawley rats with lumbar intrathecal catheters were tested for their tail withdrawal response by the tail flick test and paw flinches produced by formalin injection after intrathecal administration of saline, clonidine, AP-5 (a N-methyl-D-aspartate receptor antagonist), or YM872 (an alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor antagonist). The combinations of clonidine and the other two agents were also tested by isobolographic analyses. Motor disturbance and behavioral changes were observed as side effects. The ED(50) values of clonidine decreased from 0.26 microg (tail flick), 0.12 microg (Phase 1) and 0.13 microg (Phase 2) to 0.036 microg, 0.006 microg, and 0.013 microg with AP-5, and 0.039 microg, 0.057 microg, and 0.133 microg with YM872, respectively. Side effects were attenuated in both combinations. In conclusion, spinally administered clonidine and AP-5 or YM872 exhibited potent synergistic analgesia on acute thermal and formalin-induced nociception with decreased side effects in rats. IMPLICATIONS: Combinations of a spinally administered alpha(2) adrenergic receptor agonist and an a N-methyl-D-aspartate receptor antagonist or an alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor antagonist exhibited potent synergistic analgesia in acute thermal and inflammatory-induced nociception with decreased side effects.     

Interaction between a NMDA receptor antagonist, AP-5 and an AMPA receptor antagonist, YM 872 in antinociception in the spinal cord

Background: The intrathecal N -methyl- d -aspartate (NMDA) receptor antagonist, AP-5 and the α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor antagonist, YM 872 showed inhibition on both acute and facilitated nociception in our previous study. The present study was performed to investigate the interaction between intrathecal AP-5 and YM 872 in antinociception for acute and chronic nociception.
Methods: Sprague–Dawley rats with lumbar intrathecal catheters were tested for their thermal tail withdrawal response and for their paw flinches by formalin injection after intrathecal administration of AP-5 or YM 872. The effects of the combination were tested by an isobolographic analysis using 50% effective dose (ED50). Total fractional dose was calculated as (ED50 dose of AP-5 in combination)/(ED50 dose of AP-5 alone)+(ED50 dose of YM 872 in combination)/(ED50 dose of YM 872 alone).
Results: Intrathecally administered AP-5, YM 872, and their combination produced dose-dependent increases of the tail-flick latency and decreases in the number of flinches in both phase 1 and 2 of the formalin test. The ED50 values of the combination were significantly lower than the calculated additive values ( P <0.01). Total fractional dose value was 0.22 in the tail flick test, 0.12 in the phase 1 and 0.14 in the phase 2 of the formalin test.
Conclusion: An NMDA receptor antagonist, AP-5 and an AMPA receptor antagonist, YM 872 had synergistic antinociceptive effects on both acute thermal and inflammatory induced acute and facilitated nociception.     

The spinal antinociceptive effects of a novel competitive AMPA receptor antagonist, YM872, on thermal or formalin-induced pain in rats.

Alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor antagonists have spinally mediated analgesic effects on acute nociception; however, their current formulations are not water-soluble and have toxic side effects. A new competitive AMPA antagonist, YM872 (2,3-dioxo-7-[1H-imidazol-1-yl]-6-nitro-1,2,3,4-tetrahydro-1-quinoxal inyl acetic acid) is water-soluble and may have fewer side effects. The purpose of this study was to investigate the analgesic effects of YM872 on both acute thermal and irritant-induced pain. Sprague-Dawley rats were implanted with chronic lumbar intrathecal catheters and were tested for their tail withdrawal response by the tail flick test and for their paw flinches by formalin injection after the intrathecal administration of YM872. The tail flick latency increased dose-dependently with a 50% effective dose (ED50) value of 1.0 microg. The number of flinches in both Phase 1 and Phase 2 of the formalin test decreased with increasing dose of YM872. ED50 values were 0.24 microg in Phase 1 and 0.21 microg in Phase 2. YM872 10 and 30 microg induced motor disturbance and flaccidity. In rats, the intrathecal administration of YM872 had analgesic effects on both acute thermal and formalin-induced nociceptions. Transient motor disturbance and flaccidity occurred only with large doses. YM872 may have potential in the clinical management of both acute and chronic pain. IMPLICATIONS: A novel alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor antagonist, YM872, may have an analgesic effect on both acute and chronic pain when administered intrathecally.     

The systemically administered competitive AMPA receptor antagonist, YM872, has analgesic effects on thermal or formalin-induced pain in rats

A new competitive alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor antagonist, (2,3-dioxo-7-[1H-imidazol-1-yl]-6-nitro-1,2,3,4-tetrahydro-1-quinoxal inyl) acetic acid (YM872) has analgesic effects on acute thermal- and formalin-induced nociception by intrathecal administration. The purpose of this study was to determine the analgesic effects of systemically administered YM872 in both acute thermal- and irritant-induced pain. Sprague-Dawley rats were tested for tail withdrawal response by the tail flick test and for paw flinches by formalin injection after intraperitoneal administration of YM872. The tail flick latency increased dose-dependently with a 50% effective dose value of 156.3 microg. The number of flinches in both first and second phases of the formalin test decreased with increasing the dose of YM872. The 50% effective dose values were 1.0 microg in the first phase and 38.7 microg in the second phase. Transiently, intraperitoneal administration of 1 and 10 mg of YM872 induced motor disturbance and 10 mg induced loss of pinna reflex. We conclude that intraperitoneal administration of YM872 had analgesic effects on both acute thermal- and formalin-induced nociceptions in rats. Transient motor disturbance and loss of pinna reflex occurred only with large doses. IMPLICATIONS: Intraperitoneally administered YM872, a new alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor antagonist, had analgesic effects on thermal- and formalin-induced pain in rats. Larger doses induced transient motor disturbance and loss of pinna reflex mediated in the brain.     

Synergistic analgesic effects of intrathecal midazolam and NMDA or AMPA receptor antagonists in rats

PURPOSE: To investigate the interaction of midazolam and N-methyl-D-aspartate (NMDA) receptor or -amino-3-hydroxy-5-methyl isoxazole-4-propionic acid (AMPA) receptor antagonist on the effects of persistent inflammatory nociceptive activation. METHODS: Male Sprague-Dawley rats were implanted with lumbar intrathecal catheters and were tested for their responses to subcutaneous formalin injection into the hindpaw. Saline, midazolam (1 to 100 microg), AP-5 (I to 30 microg), a NMDA receptor antagonist, or YM872 (0.3 to 30 microg), an AMPA receptor antagonist was injected intrathecally 10 min before formalin injection. The combinations of midazolam and AP-5 or YM872 in a constant dose ratio based on the 50% effective dose (ED50) were also tested and were analysed with an isobologram. RESULTS: Dose-dependent effects were observed with midazolam (ED50 was 1.34 microg and 1.21 microg in phase 1 and 2 of the formalin test, respectively), AP-5 (7.64 microg and 1.4 microg) and YM872 (0.24 microg and 0.21 microg). Synergistic effects in both phases were obtained when combining midazolam with AP-5 or YM872. The ED50 of midazolam decreased to 0.012 microg (phase 1) and 0.27 microg (phase 2) with AP-5 and to 0.09 microg (phase 1) and 0.35 microg (phase 2) with YM872 (P < 0.01). CONCLUSIONS: These results suggest a functional coupling of benzodiazepine-aminobutyric acid (GABA)A receptor with NMDA and AMPA receptors in acute and persistent inflammatory nociceptive mechanisms in the spinal cord.     

Chronic Desipramine Treatment Desensitizes the Rat to Anesthetic and Antinociceptive Effects of the [Greek small letter alpha]2-adrenergic Agonist Dexmedetomidine

Introduction: The effects of long-term administration of the tricyclic antidepressant agent desipramine on the hypnotic, antinociceptive, anesthetic-sparing, and central norepinephrine turnover suppressant action of short-term dexmedetomidine, a highly selective [Greek small letter alpha]2-adrenergic agonist, were studied in rats.

Methods: Rats were given a 3- or 4-week course of twice daily administration of desipramine, 10 mg/kg, or saline. The effect of a hypnotic dose of dexmedetomidine, 250 [micro sign]g/kg given intraperitoneally, on the duration of loss of righting reflex was determined. The tail flick latency response was determined before and after 50 [micro sign]g/kg dexmedetomidine. The minimum anesthetic concentration of halothane and the central norepinephrine turnover rate were determined before and after administration of 30 [micro sign]g/kg dexmedetomidine. Changes in the affinity and density of the [Greek small letter alpha]2-adrenergic receptor in locus coeruleus and spinal cord also were determined.

Results: Treatment with desipramine decreased dexmedetomidine-induced loss of righting reflex duration by 67% and eliminated the antinociceptive effect of dexmedetomidine. Dexmedetomidine produced a 55% decrease in minimum anesthetic concentration in the control group but no reduction in desipramine-treated rats. Desipramine did not change the receptor density or binding affinity of [Greek small letter alpha]2 receptors at the site for hypnotic (locus coeruleus) or antinociceptive (spinal cord) responses. No decrement in the central norepinephrine turnover rate was noted in the locus coeruleus of dexmedetomidine after 3 weeks of treatment with desipramine. The [Greek small letter alpha]1-adrenergic antagonist prazosin at 1 or 5 mg/kg completely (minimum anesthetic concentration reduction), almost completely (antinociceptive), or partially (hypnotic) restored responsiveness to normal.     

Effects of Intrathecal NMDA and Non-NMDA Antagonists on Acute Thermal Nociception and Their Interaction with Morphine

Background: N-methyl-D-aspartate (NDMA) antagonists have minimal effects on acute nociception but block facilitated states of processing. In contrast, the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) antagonists decrease acute noxious responses. Morphine (a [micro sign]-opioid agonist) can also decrease acute nociceptive processing. The authors hypothesized that the interaction between morphine and AMPA receptor antagonists would be synergistic, whereas morphine and NMDA antagonists show no such interaction in acute nociception.

Methods: Sprague-Dawley rats (weight, 250-300 g) were implanted with chronic lumbar intrathecal catheters and were assigned to receive one of several doses of morphine-ACEA 1021 (NMDA glycine site antagonist), ACEA 2085 (AMPA antagonist), AP-5 (NMDA antagonist), saline or vehicle-and were tested for their effect on the response latency using a 52.5 [degree sign]C hot plate. The combinations of morphine and other agents also were tested.

Results: Intrathecal morphine (ED50:2 [micro sign]g/95% confidence interval, 1-4 [micro sign]g) and ACEA 2085 (6 ng/2-15 ng), but not AP-5 or ACEA 1021, yielded a dose-dependent increase in the thermal escape latency. A systematic isobolographic analysis was carried out between intrathecal morphine and ACEA 2085 using the ED50 dose ratio of 357:1. A potent synergy was observed with decreased side effects. Morphine dose- response curves were carried out for morphine and fixed doses of ACEA 1021 (12 [micro sign]g) or AP-5 (10 [micro sign]g). No synergistic interactions were noted.     

Stereoselective Interaction of Ketamine with Recombinant [micro sign], [small kappa, Greek], and [small delta, Greek] Opioid Receptors Expressed in Chinese Hamster Ovary Cells

Background: The authors examined the interaction of ketamine with recombinant [micro sign], [small kappa, Greek], and [small delta, Greek] opioid receptors and recombinant orphan opioid receptors expressed in Chinese hamster ovary cells (CHO-[micro sign], CHO-[Greek small letter kappa, CHO-[small delta, Greek], and CHOORL1, respectively).

Methods: CHO-[micro sign], CHO-[small kappa, Greek], and CHO-[small delta, Greek] membranes were incubated with the opioid receptor radioligand [(3) H] diprenorphine at room temperature. Ketamine (racemic, R(-) and S(+)) was included at concentrations covering the clinical range. CHOORL1 membranes were incubated with [(125) I]Tyr14 nociceptin and racemic ketamine at room temperature. The effects of racemic ketamine and selective opioid receptor agonists ([micro sign] [D-Ala2, MePhe4, Gly(ol)5] enkephalin (DAMGO); [Greek small letter kappa spiradoline or [small delta, Greek]: [D-pen2, D-pen5] enkephalin (DPDPE)) on forskolin-stimulated cyclic adenosine monophosphate formation also were examined. Data are mean +/- SEM.

Results: Racemic ketamine increased the radioligand equilibrium dissociation constant for [(3) H]diprenorphine from 85 +/- 5 to 273 +/- 11, 91 +/- 6 to 154 +/- 16, and 372 +/- 15 to 855 +/- 42 pM in CHO-[micro sign], CHO-[small kappa, Greek], AND CHO-[small delta, Greek], respectively, The concentration of radioligand bound at saturation was unaffected. In CHO-[micro sign] and CHO-[small kappa, Greek], cells, racemic, ketamine did not slow the rate of naloxone-induced [(3) H]diprenorphine dissociation. Ketamine and its isomers also displaced [(3) H]diprenorphine binding to [micro sign], [small kappa, Greek], and [small delta, Greek] receptors in a dose-dependent manner, with pKi values for racemic ketamine of 4.38 +/- 0.02, 4.55 +/- 0.04, and 3.57 +/- 0.02, respectively. S(+)-ketamine was two to three times more potent than R(-)-ketamine at [micro sign] and [small kappa, Greek] receptors. Racemic ketamine displaced [(125) I]Tyr14 nociceptin with an estimated affinity constant of 0.5 mM. Racemic ketamine inhibited the formation of cyclic adenosine monophosphate (naloxone insensitive) in a dose-dependent manner (concentration producing 50% inhibition [tilde operator] 2 mm) in all cell lines, including untransfected CHO cells. Ketamine (100 [micro sign]M) reversed DAMGO ([micro sign]) and spiradoline ([small kappa, Greek]) inhibition of formation of cyclic adenosine monophosphate.     

Effects of Intravenous General Anesthetics on [(3) H]GABA Release from Rat Cortical Synaptosomes

Background: Potentiation by general anesthetics of [Greek small letter gamma]-aminobutyric acid (GABA)-mediated inhibitory transmission in the central nervous system is attributed to GABAA receptor-medicated postsynaptic effects. However, the role of presynaptic mechanisms in general anesthetic action is not well characterized, and evidence for anesthetic effects on GABA release is controversial. The effects of several intravenous general anesthetics on [(3) H]GABA release from rat cerebrocortical synaptosomes (isolated nerve terminals) were investigated.

Methods: Purified synaptosomes were preloaded with [(3) H]GABA and superfused with buffer containing aminooxyacetic acid and nipecotic acid to inhibit GABA metabolism and reuptake, respectively. Spontaneous and elevated potassium chloride depolarization-evoked [(3) H]GABA release were evaluated in the superfusate in the absence or presence of various anesthetics, extracellular Ca2+, GABA receptor agonists and antagonists, and 2,4-diaminobutyric acid.

Results: Propofol, etomidate, pentobarbital, and alphaxalone, but not ketamine, potentiated potassium chloride-evoked [(3) H]GABA release (by 1.3 to 2.9 times) in a concentration-dependent manner, with median effective concentration values of 5.4 +/- 2.8 [micro sign]M (mean +/- SEM), 10.1 +/- 2.1 [micro sign]M, 18.8 +/- 5.8 [micro sign]M, and 4.4 +/- 2.0 [micro sign]M. Propofol also increased spontaneous [(3) H]GABA release by 1.7 times (median effective concentration = 7.1 +/- 3.4 [micro sign]M). Propofol facilitation of [(3) H]GABA release was Ca2+ dependent and inhibited by bicuculline and picrotoxin, but was insensitive to pretreatment with 2,4-diaminobutyric acid, which depletes cytoplasmic GABA pools.     

Isobolographic Analysis of the Interactions between Midazolam and Propofol at GABAA Receptors in Embryonic Mouse Neurons

Background: Clinical studies suggest that midazolam and propofol interact synergistically to induce hypnosis, but these drugs do not interact synergistically to prevent movement in response to noxious stimuli. The mechanisms underlying these interactions are not certain but may occur at the level of the [small gamma, Greek]-aminobutyric acid A (GABAA) receptor.

Methods: The authors evaluated the interactions between propofol and midazolam in modulating GABAA receptor activity in embryonic hippocampal neurons. The effects of midazolam and propofol on peak current evoked by submaximal concentrations of GABA were studied using the patch clamp method. Isobolographic analysis was undertaken by constructing concentration-response curves for midazolam and propofol alone and then evaluating the potency of combinations of midazolam and propofol. In other experiments, the concentration of GABA was increased and flurazepam was substituted for midazolam.

Results: Isobolographic analysis confirmed that midazolam and propofol interact synergistically to enhance currents evoked by low concentrations of GABA (1 [micro sign]M). However, when the concentration of GABA was increased to 3 [micro sign]M, the interaction was additive. The interaction between flurazepam and propofol was also additive for enhancement of currents evoked by 3 [micro sign]M GABA.     

Interaction of Intrathecally Infused Morphine and Lidocaine in Rats (Part II): Effects on the Development of Tolerance to Morphine

Background: There has been little information regarding the effects of local anesthetics on tolerance to opioids, although chronic use of combination of opioids and local anesthetics is popular for pain control. This study was designed to examine the effects of lidocaine on morphine tolerance to somatic and visceral antinociception.

Methods: Rats received a continuous intrathecal infusion of morphine (0.3-10 [micro sign]g [middle dot] kg-1 [middle dot] h-1), lidocaine (30-1000 [micro sign]g [middle dot] kg-1 [middle dot] h-1), a combination of those, or saline. After 6- day infusion, intrathecal morphine challenge test (5 [micro sign]g/10 [micro sign]l) was performed, and time-response curve was constructed to assess the magnitude of tolerance. The tail flick (TF) test and colorectal distension (CD) test were used to measure somatic and visceral antinociceptive effects, respectively.

Results: Antinociceptive effects in the TF and CD tests caused by morphine challenge were reduced (P < 0.01) in the morphine infused groups. The magnitude of the tolerance was inversely associated with the amount of morphine infused. Lidocaine infusion induced no different change in the morphine challenge test from that seen in the saline infusion group. Development of tolerance was greater in morphine 3 [micro sign]g [middle dot] kg-1 [middle dot] h-1 than in morphine 0.75 [micro sign]g [middle dot] kg-1 [middle dot] h-1 + lidocaine 150 [micro sign]g [middle dot] kg-1 [middle dot] h-1 despite their similar antinociceptive effects during intrathecal infusion. The infusion of a low dose of morphine (0.3 [micro sign]g [middle dot] kg-1 [middle dot] h-1) did not reduce the antinociceptive effects in the challenge test.     

Comparison of the Effects of Convulsant and Depressant Barbiturate Stereoisomers on AMPA-type Glutamate Receptors

Background: [Greek small letter alpha]-Amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-type glutamate receptors mediate fast excitatory synaptic transmission in the central nervous system. Although barbiturates have been shown to suppress the AMPA receptor-mediated responses, it is unclear whether this effect contributes to the anesthetic action of barbiturates. The authors compared the effects of depressant [R(-)] and convulsant [S(+)] stereoisomers of 1-methyl-5-phenyl-5-propyl barbituric acid (MPPB) on the AMPA and [Greek small letter gamma]-aminobutyric acid type A (GABAA) receptor-mediated currents to determine if the inhibitory effects on AMPA receptors correlate to the in vivo effects of the isomers.

Method: The authors measured whole-cell currents in the rat cultured cortical neuron at holding potential of -60 mV. Kainate 500 [micro sign]M was applied as the agonist for AMPA receptors. Thiopental (3-300 [micro sign]M), R(-)-MPPB or S(+)-MPPB (100-1,000 [micro sign]M) was coapplied with kainate under the condition in which the GABAA receptor-mediated current was blocked. Effects of MPPB isomers on the current elicited by GABA 1 [micro sign]M were studied in the separate experiments.

Results: Thiopental inhibited the kainate-induced current reversibly and in a dose-dependent manner, with a concentration for 50% inhibition of 49.3 [micro sign]M. Both R(-)-MPPB and S(+)-MPPB inhibited the kainate-induced current with a little stereoselectivity. R(-)-MPPB was slightly but significantly more potent than S(+)-MPPB. In contrast, R(-)-MPPB enhanced but S(+)-MPPB reduced the GABA-induced current.     

Interaction of Intrathecally Infused Morphine and Lidocaine in Rats (Part I): Synergistic Antinociceptive Effects

Background: Synergistic antinociception of opioids and local anesthetics has been established in bolus injections but not in long-term use. The somatic and visceral antinociceptive effects of intrathecally infused morphine or lidocaine were characterized, and the nature of the interaction of those agents in rats was evaluated.

Methods: Intrathecal catheters were implanted in rats. Morphine (0.3 to 10 [micro sign]g [middle dot] kg-1 [middle dot] h-1), lidocaine (30-1,000 [micro sign]g [middle dot] kg-1 [middle dot] h-1), a combination of those, or saline was infused intrathecally at a constant rate of 1 [micro sign]l/h for 6 days. The tail flick and colorectal distension tests were used to measure the somatic and visceral antinociceptive effects, respectively. Nociceptive tests and motor function tests were repeated on days 1, 2, 3, 4, and 6. Isobolographic analysis was performed on the results of the tail flick test to determine the magnitude of the interaction.

Results: Intrathecally infused morphine produced dose-dependent antinociceptive effects in both the tail flick and the colorectal distension tests. Morphine showed a lower peak percentage maximum possible effect (%MPE) in the colorectal distension test than in the tail flick test. Intrathecal lidocaine also produced dose-dependent antinociceptive effects. Lidocaine infusion at 1,000 [micro sign]g [middle dot] kg-1 [middle dot] h-1 caused motor impairment. Coinfusion of morphine 0.3 [micro sign]g [middle dot] kg-1 [middle dot] h-1 and lidocaine 200 [micro sign]g [middle dot] kg-1 [middle dot] h-1, which had no effects by themselves, significantly increased the percentage maximum possible effects (P < 0.01). Coinfused lidocaine potentiated the duration and the magnitude of morphine antinociception. Isobolographic analysis of the tail flick test on day 1 showed a synergistic interaction between morphine and lidocaine.     

Study of Three Different Doses of Epidural Neostigmine Coadministered with Lidocaine for Postoperative Analgesia

Background: Intrathecal neostigmine produces analgesia in volunteers and patients. However, the use of epidural neostigmine has not been investigated. The purpose of the current study was to define the analgesic effectiveness of epidural neostigmine coadministered with lidocaine and side effects in patients after minor orthopedic procedures.

Methods: After Institutional Review Board approval and informed consent, 48 patients (n = 12) undergoing knee surgery were randomly allocated to one of four groups and studied in a prospective way. After 0.05-0.1 mg/kg intravenous midazolam premedication, patients were randomized to receive 20 mg intrathecal bupivacaine plus epidural lidocaine (85 mg) with saline (control group); 1 [micro sign]g/kg epidural neostigmine (1 [micro sign]g group); 2 [micro sign]g kg epidural neostigmine (2 [micro sign]g group); or 4 [micro sign]g/kg epidural neostigmine (4 [micro sign]g group). The concept of the visual analog scale, which consisted of a 10-cm line with 0 equaling "no pain at all" and 10 equaling "the worst possible pain" was introduced. Post-operatively, pain was assessed using the visual analog scale, and intramuscular 75 mg diclofenac was available at patient request.

Results: Groups were demographically the same and did not differ in intraoperative characteristics (blood pressure, heart rate, ephedrine consumption, oxyhemoglobin saturation, sensory loss before start of surgery, or duration of sensory motor block). The visual analog scale score at first rescue analgesic and the incidence of adverse effects were similar among groups (P > 0.05). The time (min +/- SD) to first rescue analgesic was as follows: control group: 205 +/- 48; 1-[micro sign]g group: 529 +/- 314; 2-[micro sign]g group: 504 +/- 284; 4-[micro sign]g group: 547 +/- 263 (P < 0.05). The analgesic consumption (number of intramuscular diclofenac injections [mean, 25th-75th percentile]) in 24 h was as follows: control group: 3 [3 or 4]; 1-[micro sign]g group: 1[1 or 2]; 2-[micro sign]g group: 2[1 or 2]; 4-[micro sign]g group: 2[1-3](P < 0.05). The 24-h-pain visual analog scale score (cm +/- SD) that represents the overall impression for the last 24 h was as follows: control group: 5 +/- 1.6; 1-[micro sign]g group: 1.6 +/- 1.8; 2-[micro sign]g group: 1.4 +/- 1.6; 4-[micro sign]g group: 2.2 +/- 1.9 (P < 0.005). The incidence of adverse effects was similar among groups (P > 0.05).     

Nonanesthetic Volatile Drugs Obey the Meyer-Overton Correlation in Two Molecular Protein Site Models

Background: Nonanesthetic volatile compounds fail to inhibit movement in response to noxious stimulation at concentrations predicted to induce anesthesia from their oil-water partitioning. Thus they represent tools to determine whether molecular models behave like the targets that mediate in vivo anesthetic actions. The effects of volatile anesthetics and non-anesthetics were examined in two experimental models in which anesthetics interact directly with proteins: the pore of the nicotinic acetylcholine receptor and human serum albumin.

Methods: Wild-type mouse muscle nicotinic receptors and receptors containing pore mutations ([Greek small leter alpha] S252I + [Greek small letter beta] T263I) were studied electrophysiologically in membrane patches from Xenopus oocytes. Patch currents evoked by brief pulses of acetylcholine were measured in the presence of enflurane and two nonanesthetics, 1,2-dichlorohexafluorocyclobutane and 2,3-dichlorooctafluorobutane. Nonanesthetic interactions with human serum album were assessed by quenching of intrinsic protein fluorescence.

Results: Both anesthetic and nonanesthetic volatile compounds inhibited wild-type and [Greek small letter alpha] S252I + [Greek small letter beta] T263I mutant nicotinic channels but displayed different selectivity for open versus resting receptor states. Median inhibitory concentrations (IC50 s) in wild-type nicotinic receptors were 870 +/- 20 [micro sign]M for enflurane, 37 +/- 3 [micro sign]M for 1,2-dichlorohexafluorocylcobutane, and 11.3 +/- 5.6 [micro sign]M for 2,3-dichlorooctafluorobutane. For all three drugs, ratios of wild-type IC50 s to mutant IC50mut ranged from 7-10, and ratios of wild-type IC50 s to predicted anesthetic median effective concentrations (EC50 s) ranged from 1.8-2.3. 1,2-Dichlorohexafluorocyclobutane quenched human serum albumin with an apparent dissociation constant (Kd) of 160 +/- 11 [micro sign]M. The ratios of dissociation constants to predicted EC50 s for the nonanesthetics were within a factor of two of the dissociation constant:EC (50) ratios calculated for halothane and chloroform from previous published results.     

Reversal of the Sedative and Sympatholytic Effects of Dexmedetomidine with a Specific [Greek small letter alpha]2-Adrenoceptor Antagonist Atipamezole: A Pharmacodynamic and Kinetic Study in Healthy Volunteers

Background: Specific and selective [Greek small letter alpha]2-adrenergic drugs are widely exploited in veterinary anesthesiology. Because [Greek small letter alpha]2-agonists are also being introduced to human practice, the authors studied reversal of a clinically relevant dexmedetomidine dose with atipamezole, an [Greek small letter alpha]2-antagonist, in healthy persons.

Methods: The study consisted of two parts. In an open dose-finding study (part 1), the intravenous dose of atipamezole to reverse the sedative effects of 2.5 [micro sign]g/kg of dexmedetomidine given intramuscularly was determined (n = 6). Part 2 was a placebo-controlled, double-blinded, randomized cross-over study in which three doses of atipamezole (15, 50, and 150 [micro sign]g/kg given intravenously in 2 min) or saline were administered 1 h after dexmedetomidine at 1-week intervals (n = 8). Subjective vigilance and anxiety, psychomotor performance, hemodynamics, and saliva secretion were determined, and plasma catecholamines and serum drug concentrations were measured for 7 h.

Results: The mean +/- SD atipamezole dose needed in part 1 was 104 +/- 44 [micro sign]g/kg. In part 2, dexmedetomidine induced clear impairments of vigilance and psychomotor performance that were dose dependently reversed by atipamezole (P < 0.001). Complete resolution of sedation was evident after the highest (150 [micro sign]g/kg) dose, and the degree of vigilance remained high for 7 h. Atipamezole dose dependently reversed the reductions in blood pressure (P < 0.001) and heart rate (P = 0.009). Changes in saliva secretion and plasma catecholamines were similarly biphasic (i.e., they decreased after dexmedetomidine followed by dose-dependent restoration after atipamezole). Plasma norepinephrine levels were, however, increased considerably after the 150 [micro sign]g/kg dose of atipamezole. The pharmacokinetics of atipamezole were linear, and elimination half-lives for both drugs were approximately 2 h. Atipamezole did not affect the disposition of dexmedetomidine. One person had symptomatic sinus arrest, and another had transient bradycardia approximately 3 h after receiving dexmedetomidine.     

The effects of an AMPA receptor antagonist on the neurotoxicity of tetracaine intrathecally administered in rabbits

We have reported that large concentrations of intrathecal local anesthetics increase glutamate concentrations in the cerebrospinal fluid (CSF) and cause neuronal injury in rabbits. In the current study we determined whether an alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor antagonist, YM872, administered intrathecally, reduces neuronal injury caused by tetracaine. We first examined the effects of intrathecal YM872 10, 30, 100, or 300 mug in rabbits (n = 3 in each). YM872 produced reversible motor and sensory block in a dose-dependent manner. Then, we evaluated modulatory effects of YM872 (300 mug) on tetracaine-induced glutamate release and neuronal injury. Pretreatment of YM872 did not attenuate 1% or 2% tetracaine-induced increases in cerebrospinal fluid glutamate concentrations (n = 3 in each). For evaluation of neuronal injury, rabbits were assigned to 4 groups (n = 6 in each) and intrathecally received 1% tetracaine and saline (1%T), 1% tetracaine and YM872 (1%TY), 2% tetracaine and saline (2%T), or 2% tetracaine and YM872 (2%TY). The volume of saline, YM872, and tetracaine was 0.3 mL. Saline or YM872 was administered 30 min before tetracaine administration. Neurological and histopathological assessments were performed 1 wk after the administration. Two and 1 animals respectively, showed motor and sensory dysfunction in 1%T, whereas 5 animals showed both motor and sensory dysfunction in 2%T. YM872 improved 2% tetracaine-induced motor dysfunction and neuronal damage (chromatolytic neurons, identified by round-shaped cytoplasm with loss of Nissl substance from the central part of the cell and eccentric nuclei). In 2%TY, 3 animals showed normal motor function and 3 showed mild dysfunction (ability to hop, but not normally), whereas 4 animals showed moderate dysfunction (inability to hop) in 2%T (P = 0.042). Only 2 animals showed one chromatolytic neuron in 2%TY, whereas 5 animals showed 4-16 chromatolytic neurons in 2%T (P = 0.020). These results suggest that AMPA receptor activation is involved, at least in part, in the tetracaine-induced neurotoxicity in the spinal cord.     

Neurokinin-1 Receptors Are Involved in Behavioral Responses to High-intensity Heat Stimuli and Capsaicin-induced Hyperalgesia in Mice

Background: The neurokinin-1 (NK-1) receptor and its ligand, substance P, are thought to play important roles in nociception and hyperalgesia. This study evaluated the role of the NK-1 receptor in processing noxious stimuli in normal and inflammatory states.

Methods: Behavioral responses to heat and mechanical and chemical stimuli were studied in NK-1 receptor knockout mice and wild-type control mice. Thermal nociception was evaluated by measuring paw lick or jump latencies to hot plate (52, 55, and 58[degree sign]C) and paw withdrawal latencies to radiant heat applied to the hind paws. Mechanical nociception was measured by von Frey monofilament applications to the hind paws. Intraplantar capsaicin-induced (10 [micro sign]g/20 [micro sign]l) paw licking and mechanical and heat hyperalgesia were compared in NK-1 knockout and wild-type mice.

Results: Withdrawal responses to radiant heat (4.3 +/- 0.18 s for knockout and 4.4 +/- 0.8 s for wild-type mice) and von Frey monofilaments were similar in knockout and wild-type mice. In the hot plate test, increasing the hot plate temperature from 52[degree sign]C to 58[degree sign]C resulted in a decrease in the response latency in the wild-type mice (30.4 +/- 17.5 s to 15.2 +/- 6.8 s, P < 0.05), whereas in the knockout mice the response latencies remained constant (28.2 +/- 19.8 s to 29 +/- 15.1 s, not significant). Capsaicin-induced paw licking (14.5 +/- 12.8 s for knockout and 41.3 +/- 37.3 s for wild-type mice, P < 0.05) and mechanical and heat hyperalgesia were attenuated in the knockout mice.     

MAC reduction after intrathecal coadministration of GABA(A) agonist and glutamate antagonist in rats

Purpose. It has been reported that intrathecal coadministration of a GABA_A agonist and a glutamate antagonist induces synergistic antinociceptive effects in rats. We hypothesized that this synergistic antinociceptive effect might induce a synergistic reduction in the minimum alveolar anesthetic concentration (MAC). Methods. The MAC for sevoflurane was determined before and after intrathecal administration of muscimol (GABA_A agonist, 0.1–10 μg), AP-5 (N-methyl-d-aspartate [NMDA] antagonist, 0.1–10 μg), and YM872 (α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid [AMPA] antagonist, 0.1–10 μg) in rats. The effects of coadministration of muscimol and AP-5 or YM872 on MAC reduction were also tested. Results. Intrathecal administration of muscimol at doses of 1 and 10 μg significantly reduced the MAC by 34.5% ± 3.3% and 46.9% ± 4.4%, respectively (P < 0.01). Intrathecal administration of AP-5 at a dose of 10 μg and YM872 at a dose of 10 μg significantly reduced the MAC by 25.8% ± 2.4% and 31.4% ± 6.3%, respectively (P < 0.01). No additional reductions in MAC values were observed when a GABA_A agonist was combined with either an NMDA or an AMPA antagonist. Conclusion. Intrathecal coadministration of a GABA_A agonist and an NMDA or AMPA antagonist, which has been reported to produce a synergistic antinociceptive effect, did not induce a synergistic reduction in the MAC. The antinociceptive effect may not be the predominant factor in the reduction of the MAC induced by these drugs. Received: May 24, 2002 / Accepted: November 20, 2002 Acknowledgments. YM872 was supplied by Yamanouchi Pharmaceutical, Tsukuba, Japan. Financial support was provided solely from institutional and departmental sources. The authors wish to thank Ms. Masako Tachibana for her skillful technical help. Address correspondence to: H. Sumikura