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 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.     

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.     

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.     

Analgesic Interaction between Intrathecal Midazolam and Glutamate Receptor Antagonists on Thermal-induced Pain in Rats

Background: Two major neurotransmitters, [Greek small letter 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-GABAA 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 [micro sign]g), AP-5 (1-30 [micro sign]g), or YM872 (0.3-30 [micro sign]g). AP-5 is an N-methyl-D-aspartate (NMDA) receptor antagonist and YM872 is an [Greek small letter 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) [micro sign]g, 5.54 +/- 0.19 [micro sign]g, and 1.0 +/- 0.22 [micro sign]g, respectively. A potent synergy in analgesia with decreased behavioral changes and motor disturbance was obtained when combining midazolam with AP-5 or YM872.     

Analgesic effects of intrathecally administered celecoxib, a cyclooxygenase-2 inhibitor, in the tail flick test and the formalin test in rats

BACKGROUND: The analgesic effects of celecoxib, a cyclooxygenase-2 (COX-2) inhibitor, on formalin-induced pain are still controversial. The purpose of this study was to examine the analgesic effects of intrathecally administered celecoxib on inflammatory induced pain, thermal-induced pain and hemodynamics. METHODS: Male Sprague-Dawley rats with lumbar intrathecal catheters were tested via their tail withdrawal response to thermal stimulation (tail flick test) and via their paw flinching and shaking response to subcutaneous formalin injection into the hind paw (formalin test) after intrathecal administration of celecoxib. The blood pressure, pulse rate and behavioral side-effects were also examined. RESULTS: Even the maximum available dose of celecoxib (200 microg/20 microl) had little effect in the tail flick test. In the formalin test, celecoxib induced a dose-dependent decrease in the flinch response in both phases 1 and 2. The 50% effective doses were 0.025 microg (95% confidence interval, 0.007-0.082 microg) in phase 1, 0.026 microg (0.010-0.045 microg) in phase 2a and 0.001 microg (0.00009-0.010 microg) in phase 2b. With the doses used in this study, the blood pressure and pulse rate did not change and no motor disturbance or behavioral side-effects were observed. CONCLUSION: Intrathecal administration of celecoxib decreased inflammatory acute and facilitated pain without any hemodynamic or behavioral side-effects, but had no effect on acute thermal pain.     

Intrathecal propofol has analgesic effects on inflammation-induced pain in rats

PURPOSE: Propofol is thought to act on gamma-aminobutyric acid receptors, which have some role in pain transmission in the spinal cord. In this study, we examined the effects of intrathecal propofol on acute thermally- or inflammation-induced pain in rats. METHODS: Lumbar intrathecal catheters were implanted in Male Sprague-Dawley rats. The tail withdrawal response to thermal stimulation (tail flick test) or paw flinching and shaking response by sc formalin injection into the hind paw (formalin test) were tested. Propofol 1000, 300 or 100 microg or saline (control) was administered as 10 microL intrathecally. Motor disturbance and behavioural side effects were also monitored in the rats during the tail flick test. Eight rats were used for each dose in each test. RESULTS: No analgesic effects were observed in the tail flick test. In the formalin test, 50% of effective doses were 449 mug (95% confidence interval, 80-3180 microg) in phase 1 and 275 microg (146-519 microg) in phase 2. Motor disturbance was observed in one rat with 100 microg and agitation and allodynia were seen in one rat with 300 microg. However, both were reversible in 120 min. CONCLUSIONS: Intrathecal administration of propofol had analgesic effects on inflammation-induced acute and facilitated pain but not on thermally-induced acute pain. Transient motor and sensory disturbance could not rule out the possibility of neurotoxicity.     

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.     

Analgesic effects of systemic midazolam: comparison with intrathecal administration

PURPOSE: Midazolam has antinociceptive effects when administered intrathecally, while its effects associated with systemic administration remain controversial. In the present study, the antinociceptive properties of systemically vs intrathecally administered midazolam were investigated in a rat model of thermal and inflammatory pain. METHODS: One hundred seventy-six (n = 8 animals per dose escalation) male Sprague-Dawley rats were instrumented with lumbar intrathecal catheters. Tail withdrawal in response to thermal stimulation, or paw flinching and shaking in response to sc hind paw formalin injection were compared following intrathecal injection of midazolam (1, 3, 10, 30, or 100 microg in 10 microL) or ip administration (3, 30, 300, or 3,000 microg in 300 microL). Saline 10 microL or 300 microL was used as a control. Behavioural side effects and motor disturbance were also examined. RESULTS: Intrathecal administration of midazolam increased tail flick latency dose dependently (P < 0.05) with a 50% effective dose (ED50) of 1.60 microg, whereas ip administration did not increase latency. Both intrathecal and ip routes of administration decreased the number of paw flinches in both phases 1 and 2 of the formalin test (P < 0.05). The ED50s were 1.26 microg [confidence interval (CI), 0.35-3.18 microg], (phase 1) and 1.20 microg (CI, 0.29-3.71 microg), (phase 2) with intrathecal administration, and 11.6 microg (CI, 2.5-19.3 microg), (phase 1) and 52.2 microg (CI, 18.3-102.7 microg), (phase 2) with ip administration. CONCLUSION: Systemically administered midazolam induced antinociception for inflammatory pain only, while intrathecal administration elicited antinociceptive effects on both acute thermal and inflammatory-induced pain.     

Interaction between midazolam and serotonin in spinally mediated antinociception in rats

Purpose  Intrathecal administration of serotonin (5-HT) is antinociceptive through the involvement of spinal cord γ-aminobutyric acid (GABA) receptors. Therefore, 5-HT would interact with the GABA agonist, midazolam, which is well known to exert spinally mediated antinociception in the spinal cord. The present study investigated the antinociceptive interaction between spinally administered 5-HT and midazolam, using two different rat nociceptive models. Methods  Sprague-Dawley rats with lumbar intrathecal catheters were tested for their thermal tail withdrawal response and paw flinches induced by formalin injection after the intrathecal administration of midazolam or 5-HT, or the midazolam/ HT combination. The effects of the combination were tested by isobolographic analysis, using the combination of each 1, 1/2, 1/4, 1/8, and 1/16 of the 50% effective dose (ED50). The total fractional dose was calculated. Behavioral side effects were also examined. Results  5-HT alone and midazolam alone both showed dose-dependent antinociception in both the tail flick test and the formalin test. The ED50 of the combination was not different from the calculated additive value either in the tail flick test or in phase 2 of the formalin test, but it was significantly smaller than the calculated additive value in phase 1 of the formalin test. The total fractional dose value was 0.90 in the tail flick test, 0.093 in phase 1 of the formalin test, and 1.38 in phase 2 of the formalin test. The agitation, allodynia, or motor disturbance observed with either agent alone was not seen with the combination treatment. Conclusion  The antinociceptive effects of intrathecal midazolam and 5-HT were additive on thermal acute and inflammatory facilitated stimuli, and synergistic on inflammatory acute stimulation.     

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.     

Interaction among NMDA receptor-, NMDA glycine site- and AMPA receptor antagonists in spinally mediated analgesia

PURPOSE: The NMDA (N-methyl-D-aspartate) receptor antagonists and the NMDA glycine site antagonists given alone have minimal effects on acute nociception. In contrast, the AMPA (alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid) receptor antagonists have a major role in acute nociception. We investigated the interactions among these three antagonists in acute nociception. METHODS: Sprague-Dawley rats (250-300 g) were implanted with chronic lumbar intrathecal catheters and were tested for their thermal withdrawal response using the hot plate test after intrathecal administration of AP-5 (NMDA receptor antagonist), ACEA 1021 (NMDA glycine site antagonist), or ACEA 2085 (AMPA receptor antagonist). The combinations of these three agents were also tested. RESULTS: Intrathecal administration of ACEA 2085 had a dose dependent analgesic effect while intrathecal AP-5 or ACEA 1021 could not induce dose dependent effect. Co-administration of AP-5 10 microg and ACEA 2085 intrathecally showed no changes in the thermal response latency compared with ACEA 2085 alone. ACEA 1021, 12 microg, and AP-5 showed left-ward shift of the dose effect curve only with small doses of AP-5 (1 microg, 3 microg). Only the smallest dose of ACEA 2085 (0.1 ng) with ACEA 1021 12 microg induced antinociception compared with that of ACEA 2085 alone. CONCLUSIONS: The combination of the NMDA glycine site antagonist and low doses of the NMDA receptor antagonist or the AMPA receptor antagonist increased the analgesic effect on acute thermal nociception with increased side effects, while the NMDA receptor antagonist and the AMPA receptor antagonist had no such interaction.     

Intrathecal clonidine and bupivacaine have synergistic analgesia for acute thermally or inflammatory-induced pain in rats

We investigated the interaction between spinally administered bupivacaine and clonidine using an animal model of acute and inflammatory pain. Rats implanted with lumbar intrathecal catheters were injected intrathecally with saline (control), bupivacaine (1 to 100 microg), or clonidine (0.1 to 3 microg) and tested for their responses to thermal stimulation to the tail (tail flick test) and subcutaneous formalin injection into the hindpaw (formalin test). The effects of the combination of bupivacaine and clonidine on both stimuli were tested by isobolographic analysis. General behavior and motor function were examined as side effects. The 50% effective doses of bupivacaine and clonidine were significantly smaller when combined compared with each single drug in both the tail flick test (2.82 and 0.11 microg versus 7.1 and 0.29 microg, respectively) and phase 1 (0.24 and 0.009 microg versus 5.7 and 0.15 microg) and phase 2 (0.31 and 0.012 microg versus 3.2 and 0.16 microg) of the formalin test. Side effects were decreased by the combination. These results suggest a favorable combination of intrathecal bupivacaine and clonidine in the management of acute and inflammatory pain. IMPLICATIONS: The analgesic interaction between intrathecally administered bupivacaine and clonidine was examined during acute thermal and inflammatory-induced pain in rats. The analgesia produced by the combination of these two drugs was synergistic in both acute thermal and inflammatory induced pain, with a decrease in behavioral side effects.     

Spinal neurotoxicity and tolerance after repeated intrathecal administration of YM 872, an AMPA receptor antagonist,in rats

Purpose Although the -amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor antagonist, YM 872, has been considered to be useful in analgesia for both acute and chronic pain, there are no studies of its neurotoxicity and tolerance. We examined the spinal neurotoxicity and tolerance of YM 872 analgesia by repeated intrathecal administration in rats.Methods Male Sprague-Dawley rats with lumbar intrathecal catheters received YM 872 at 1µg·10µl^–1 (eight rats; YM group) or normal saline 10µl (eight rats; C group) intrathecally once a day for 30 days. We evaluated the analgesic effects every 3 days, by tail-flick test and behavioral side effects. On the 31st day, the lumbar spinal cord was removed from four randomly selected rats in each group for histological examination.Results The YM group showed significantly longer tail-flick latency when subjected to a high-intensity light beam than the C group at each measurement time point, although no significant changes in the latency according to the time course of the study were observed for the entire study period of 30 days in either group. No rats showed any side effects. Histologically, only slight lymphocytic cell infiltration and degeneration of myelinated fibers occurred, similarly in both groups. No changes were observed in the spinal cord in either group.Conclusion Administration of YM 872 (1µg) once a day for 30 days did not induce any tolerance and caused no histological changes in the spinal cord.     

Midazolam can potentiate the analgesic effects of intrathecal bupivacaine on thermal- or inflammatory-induced pain

Epidurally administered midazolam can potentiate analgesia by epidural bupivacaine. However, whether this effect is synergistic or additive is not known. In this study, we investigated the spinally-mediated analgesic interaction between midazolam and bupivacaine by using the tail-flick and formalin tests in rats with chronically implanted catheters. Behavioral effects were also observed. The dose dependency of analgesia and the 50% effective doses of intrathecal midazolam and bupivacaine were determined, and then the interaction of these two drugs was examined with an isobolographic analysis. Both drugs had dose-dependent analgesic effects in both the tail-flick test and the formalin test. The 50% effective dose values of the combination were significantly lower than the calculated additive values in both tests (P = 0.023 in the tail-flick test; P = 0.0025 in Phase 1 and 0.047 in Phase 2 of the formalin test). Behavioral side effects decreased in the combination group compared with each drug alone. In conclusion, intrathecally administered midazolam and bupivacaine had synergistic analgesic effects on acute thermal- or inflammatory-induced pain, with decreased behavioral side effects. IMPLICATIONS: In both acute thermal- and inflammatory-induced pain, intrathecally administered midazolam and bupivacaine produced synergistic analgesia with decreased side effects in intrathecally catheterized rats.     

The synergistic interaction between midazolam and clonidine in spinally-mediated analgesia in two different pain models of rats

Both midazolam, a benzodiazepine gamma-aminobutyric acid type A receptor agonist, and clonidine, an alpha2-adrenergic receptor agonist, induce spinally-mediated analgesia. We investigated the analgesic interaction of spinally-administered midazolam and clonidine in their effects on acute and inflammatory nociception. Rats implanted with lumbar intrathecal catheters were injected intrathecally with saline (control), midazolam (1 to 100 microg), or clonidine (0.1 to 3 microg) to test for their responses to thermal stimulation to the tail (tail-flick test) and subcutaneous formalin injection into the hind paw (formalin test). The effects of the combination of midazolam and clonidine on both stimuli were tested by isobolographic analysis by using the 50% effective doses. The general behavior and motor function were examined as side effects. When combined, the 50% effective doses of midazolam (clonidine) decreased from 1.57 microg (0.26 microg) to 0.29 g (0.05 microg) in the tail-flick test and from 1.34 microg (0.12 microg) and 1.21 microg (0.13 microg) to 0.05 microg (0.005 microg) and 0.13 microg (0.015 microg) in Phase 1 and 2 of the formalin test, respectively. Side effects did not increase by using the combination. These results suggest a favorable combination of intrathecal midazolam and clonidine in the management of acute and inflammatory pain after proper neurotoxicologic studies. IMPLICATIONS: Spinally-administered midazolam, a benzodiazepine, and clonidine, an alpha2-adrenergic receptor agonist, have significant synergistic effects on thermally-induced acute and formalin-induced inflammatory pain.     

Effects of radolmidine, a novel alpha2 -adrenergic agonist compared with dexmedetomidine in different pain models in the rat

BACKGROUND: Intrathecally administered alpha2-adrenoceptor agonists produce effective antinociception, but sedation is an important adverse effect. Radolmidine is a novel alpha2-adrenoceptor agonist with a different pharmacokinetic profile compared with the well-researched dexmedetomidine. This study determined the antinociceptive and sedative effects of radolmidine in different models of acute and chronic pain. Dexmedetomidine and saline served as controls. METHODS: Male Sprague-Dawley rats were studied in acute pain (tail flick), carrageenan inflammation, and the spinal nerve ligation model of neuropathic pain. Mechanical allodynia was assessed with von Frey filaments, cold allodynia with the acetone test, and thermal hyperalgesia with the paw flick test. Locomotor activity-vigilance was assessed in a dark field. Dexmedetomidine and radolmidine were administered intrathecally in doses of 0.25 microg, 2.5 microg, 5 microg, and 10 microg. RESULTS: In the tail flick test, radolmidine showed a dose-dependent antinociceptive effect, being equipotent compared with dexmedetomidine. In carrageenan inflammation, intrathecal doses of 2.5 microg or 5 microg of dexmedetomidine/radolmidine produced significant antinociception compared with saline (P < 0.01). The two drugs were equianalgesic. In the neuropathic pain model, an intrathecal dose of 5 microg dexmedetomidine-radolmidine had a significant antiallodynic effect compared with saline (P < 0.01). The two drugs were equipotent. Intrathecal administration of both dexmedetomidine and radolmidine dose dependently decreased spontaneous locomotor acitivity-vigilance, but this effect was significantly smaller after intrathecal administration of radolmidine than after intrathecal dexmedetomidine. CONCLUSIONS: Radolmidine and dexmedetomidine had equipotent antinociceptive effects in all tests studied. However, radolmidine caused significantly less sedation than dexmedetomidine, probably because of a different pharmacokinetic profile.     

Intrathecal edaravone, a free radical scavenger, is effective on inflammatory-induced pain in rats

BACKGROUND: Free radicals have some roles in inflammation and systemic and local tissue injuries. (Free radical scavengers are neuroprotective against excitotoxic insults.) Therefore, we hypothesized that free radical scavenger would be analgesic on pain induced by excitotoxicity or inflammation. The purpose of this study was to investigate analgesic effects of intrathecally administered edaravone, a free radical scavenger, on thermal and inflammatory pain. METHODS: Sprague-Dawley rats were implanted with lumbar intrathecal catheters. Edaravone 0.05, 0.1, 0.5, and 1 mg per 20 microl or saline 20 microl (control) were administered intrathecally, and the withdrawal response to thermal stimulation to the tail (tail-flick test) or flinch responses to subcutaneous formalin injection into the hind paw (formalin test) were tested. General behaviour and motor function were also examined. In each dose group, eight rats were used. RESULTS: No dose-dependent analgesic effects were observed in the tail-flick test. However, dose-dependent analgesia was obtained in both phase 1 and 2 of the formalin test. The 50% effective dose values were 0.25 mg (95% confidence interval, 0.11-0.56 mg) in phase 1 and 0.25 mg (95% confidence interval, 0.061-1.05 mg) in phase 2. No behavioural side-effects nor motor dysfunction was observed, even with the maximum soluble dose (1 mg/20 microl). CONCLUSION: Intrathecally administered edaravone, a free radical scavenger, had analgesic effects on inflammatory-induced acute and facilitated pain but not on acute thermal pain, without any behavioural side-effects.     

The effects of intrathecal morphine encapsulated in L- and D-dipalmitoylphosphatidyl choline liposomes on acute nociception in rats

Liposomes can serve as a sustained-release carrier system, permitting the spinal delivery of large opioid doses restricting the dose for acute systemic uptake. We evaluated the antinociceptive effects of morphine encapsulated in liposomes of two isomeric phospholipids, L-dipalmitoylphosphatidyl choline (L-DPPC) and D-dipalmitoylphosphatidyl choline (D-DPPC), in comparison with morphine in saline. Sprague-Dawley rats with chronic lumbar intrathecal catheters were tested for their acute nociceptive response using a hindpaw thermal escape test. Their general behavior, motor function, pinna reflex, and corneal reflex were also examined. The duration of antinociception was longer in both liposomal morphine groups than in the free morphine group. The peak antinociceptive effects were observed within 30 min after intrathecal morphine, L-DPPC or D-DPPC morphine injection. The rank order of the area under the effect-time curve for antinociception was L-DPPC morphine > D-DPPC morphine > morphine. The 50% effective dose was: 2.7 microg (morphine), 4.6 microg (L-DPPC morphine), and 6.4 microg (D-DPPC morphine). D-DPPC morphine had less side effects for a given antinociceptive AUC than morphine. In conclusion, L-DPPC and D-DPPC liposome encapsulation of morphine prolonged the antinociceptive effect on acute thermal stimulation and could decrease side effects, compared with morphine alone. Implications: Two isomers of liposome (L-dipalmitoylphosphatidyl choline and D-dipalmitoylphosphatidyl choline) encapsulation of morphine prolonged the analgesic effect on acute thermal-induced pain when administered intrathecally and could decrease side effects, compared with morphine alone.     

Pain models display differential sensitivity to Ca2+-permeable non-NMDA glutamate receptor antagonists

BACKGROUND: Ca2+-permeable non-N-methyl-D-aspartate receptors are found in the spinal dorsal horn and represent a presumptive target for glutamatergic transmission in nociceptive processing. This study characterized the analgesic profile associated with the blockade of these spinal receptors by intrathecally delivered agents known to act at these receptors, the spider venom Joro toxin (JST) and philanthotoxin. METHODS: Philanthotoxin (0.5, 2.5, or 5 microg) or JST (5 microg) was given spinally before thermal injury to the paw. JST (5 microg) was also given 10 min before subcutaneous formalin injection, after intraplantar administration of carrageenan, and to rats that were allodynic due to tight ligation of spinal nerves. Lower doses of JST (0.25 and 1.0 microg) were given before formalin injection and testing of thermal latencies. Thermal latencies were measured using a Hargreaves box, mechanical thresholds using von Frey hairs, and formalin response by means of counting flinches. RESULTS: Both agents blocked thermal injury-induced mechanical allodynia. JST (5 microg) given 1 h after carrageenan blocked induction of thermal hyperalgesia and mechanical allodynia. JST (5 microg) had no effect in the formalin test, on allodynia after spinal nerve ligation, or when given 3 h after carrageenan. The lowest dose (0.25 microg JST) at pretreatment intervals of 60-120 min resulted in modest hypoalgesia during phase 1 formalin and thermal testing. CONCLUSIONS: The behavioral effect of intrathecal Ca2+-permeable non-N-methyl-D-aspartate antagonists indicates an important role for this spinal receptor in regulating hyperalgesic states induced by tissue injury and inflammation and reveals an action that is distinct from those observed with other glutamate receptor antagonists.