Systemic physostigmine shows antiallodynic effects in neuropathic rats.

The aim of this study was to examine the antiallodynic and antinociceptive effects of subcutaneously administered physostigmine (50, 100, 200 micrograms/kg), compared with morphine (2.5, 5, 10 mg/kg) and NaCl after spinal nerve ligation in rats. The following stimuli were used: acetone (cold allodynia), von Frey hairs (mechanical allodynia), and paw flick test (thermal nociception). Motility boxes were used to investigate the effects of the drugs on motor performance. Physostigmine attenuated both mechanical and cold allodynia dose-dependently but had no effect on the paw flick test. The effect was antagonized by atropine (muscarinic receptor antagonist) but not by mecamylamine (nicotinic receptor antagonist) or naloxone (opioid receptor antagonist). Morphine produced dose-dependent antiallodynic and antinociceptive effects. In the antiallodynic doses, morphine caused severe rigidity. Physostigmine 200 micrograms/kg impaired locomotor activity, but no rigidity was observed. Implications: Physostigmine has different effects on allodynia and nociception, which suggests that different cholinergic (muscarinic) mechanisms may be involved in neuropathic and nociceptive pain.     

Nerve injury induces a tonic bilateral mu-opioid receptor-mediated inhibitory effect on mechanical allodynia in mice

BACKGROUND: Mice lacking the mu-opioid receptor gene have been used to characterize the role of mu-opioid receptors in nociception and the analgesic actions of opioid agonists. In this study, the authors determined the role of mu-opioid receptors in neuropathic pain behaviors and the effectiveness of mu- and kappa-opioid receptor agonists on this behavior in mice. METHODS: The authors studied the behavioral responses of mu-opioid receptor knockout and wild-type mice to thermal and mechanical stimuli before and after neuropathic pain induced by unilateral ligation and section of the L5 spinal nerve. Response to mechanical stimuli was evaluated by determining the frequency of hind paw withdrawal to repetitive stimulation using a series of von Frey monofilaments. Thermal hyperalgesia was assessed by determining the paw withdrawal latencies to radiant heat and frequency of hind paw withdrawal to cooling stimuli. The effects of systemic morphine, the kappa-opioid agonist U50488H, and naloxone on responses to mechanical and thermal stimuli were also studied in spinal nerve-injured mice. RESULTS: After spinal nerve injury, wild-type mice developed increased responsiveness to mechanical, heat, and cooling stimuli ipsilateral to nerve injury. mu-Opioid receptor knockout mice not only had more prominent mechanical allodynia in the nerve-injured paw, but also expressed contralateral allodynia to mechanical stimuli. Hyperalgesia to thermal stimuli was similar between mu-opioid knockout and wild-type animals. Morphine decreased mechanical allodynia dose dependently (3-30 mg/kg subcutaneous) in wild-type mice--an effect that was attenuated in the heterozygous mice and absent in the homozygous mu-opioid knockout mice. The kappa-opioid agonist U50488H (3-10 mg/kg subcutaneous) attenuated mechanical allodynia in wild-type, heterozygous, and homozygous mu-opioid mice. Naloxone in wild-type mice resulted in enhanced ipsilateral and contralateral allodynia to mechanical stimuli that resembled the pain behavior observed in mu-opioid receptor knockout mice. CONCLUSIONS: The authors' observations indicate that (1) unilateral nerve injury induces a bilateral tonic activation of endogenous mu-opioid receptor-mediated inhibition that attenuates mechanical allodynia but not thermal hyperalgesia, (2) both mu- and kappa-opioid agonists attenuate neuropathic pain in mice, and (3) the antihyperalgesic actions of morphine are mediated primarily via mu-opioid receptors.     

Opioid Self-administration in the Nerve-injured Rat: Relevance of Antiallodynic Effects to Drug Consumption and Effects of Intrathecal Analgesics

Background: Neuropathic pain is associated with several sensory abnormalities, including allodynia as well as spontaneous pain. Opioid intake in neuropathic pain patients is motivated by alleviation of both pain and allodynia. However, laboratory animal studies rely almost exclusively on reflexive withdrawal measures of allodynia. The authors examined the pharmacology of self-regulated intake of opioids in rats with or without nerve injury and compared the rate of drug intake to reversal of allodynia.

Methods: Rats were implanted with intravenous catheters, and the L5 and L6 spinal nerves were ligated in half of these animals. Rats were then trained to self-administer a commonly abused opioid (heroin) and commonly prescribed opioids (morphine, fentanyl, hydromorphone, and methadone). In addition, rats trained to self-administer heroin were given either clonidine or adenosine spinally before self-administration sessions to assess opioid-sparing effects.

Results: Nerve injury significantly decreased the reinforcing effects of low doses of opioids, and only doses of each opioid that reduced mechanical hypersensitivity maintained self-administration after spinal nerve ligation. The rate of drug consumption was correlated with the duration of the antiallodynic effect for each dose of opioid. Intrathecal administration of clonidine or adenosine reversed mechanical hypersensitivity, but only clonidine reduced heroin self-administration in rats with spinal nerve ligation.     

Morphine can enhance the antiallodynic effect of intrathecal R-PIA in rats with nerve ligation injury

Nerve ligation injury may produce a tactile allodynia. Intrathecal adenosine receptor agonists or morphine have an antiallodynic effect. In this study, we examined the effect of intrathecal morphine on the antiallodynic state induced by the adenosine A1 receptor agonist, N(6)-(2-phenylisopropyl)-adenosine R-(-)isomer (R-PIA), in a rat model of nerve ligation injury. Rats were prepared with ligation of left L5-6 spinal nerves and intrathecal catheter implantation. Tactile allodynia was measured by applying von Frey filaments to the lesioned hindpaw. Thresholds for withdrawal response were assessed. Morphine and R-PIA were administered to obtain the dose-response curve and the 50% effective dose (ED(50)). Fractions of ED(50)s were administered concurrently to establish the ED(50) of the drug combination. The drug interaction was analyzed using the isobolographic method. Intrathecal 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), an A1 receptor antagonist, and naloxone were administered to examine the reversal of the antiallodynic effect. Side effects were also observed. Intrathecal morphine and R-PIA and their combination produced a dose-dependent antagonism without severe side effects. Intrathecal morphine synergistically enhanced the antiallodynic effect of R-PIA when coadministered. Intrathecal naloxone and DPCPX reversed the maximal antiallodynic effect in the combination group. These results suggest that activation of mu-opioid and A1 receptors at the spinal level is required for the synergistic interaction on tactile allodynia.     

An analysis of drug interaction between morphine and neostigmine in rats with nerve-ligation injury

Intrathecal neostigmine reverses mechanical allodynia in humans and animals. The efficacy of morphine in a neuropathic pain state is still controversial. This study examines the antiallodynic interaction between morphine and neostigmine in a rat model of neuropathic pain. Rats were prepared with tight ligation of left L5-6 (fifth and sixth lumbar) spinal nerves and chronic intrathecal catheter implantation. Mechanical allodynia was measured by using application of von Frey hairs to the left hindpaw. Morphine (1, 3, 10, and 30 microg) and neostigmine (0.3, 1, 3, and 10 microg) were administered intrathecally to obtain the dose-response curves and the 50% effective dose (ED(50)) for each drug. ED(50) values and fractions of the ED(50) values (1/2, 1/4, and 1/8) were administered intrathecally in an equal dose ratio to establish the ED(50). Isobolographic and fractional analyses for the drug interaction were performed. Intrathecal morphine produced a moderate antagonism of the tactile allodynia. A morphine-neostigmine combination produced a dose-dependent increase in withdrawal threshold of the lesioned hind paw with reduced side effects. Both analyses revealed a synergistic interaction after the coadministration of morphine and neostigmine. These experiments suggest that the antiallodynic action of a morphine-neostigmine combination is synergistic at the spinal level. IMPLICATIONS: This study indicates that, by using both isobolographic and fractional analyses, the antiallodynic effect of intrathecal morphine and neostigmine is synergistic when coadministered intrathecally. In a rat model of neuropathic pain, the intrathecal morphine produced a moderate antagonism on touch-evoked allodynia at the spinal level.     

Opioid self-administration in the nerve-injured rat: relevance of antiallodynic effects to drug consumption and effects of intrathecal analgesics

BACKGROUND: Neuropathic pain is associated with several sensory abnormalities, including allodynia as well as spontaneous pain. Opioid intake in neuropathic pain patients is motivated by alleviation of both pain and allodynia. However, laboratory animal studies rely almost exclusively on reflexive withdrawal measures of allodynia. The authors examined the pharmacology of self-regulated intake of opioids in rats with or without nerve injury and compared the rate of drug intake to reversal of allodynia. METHODS: Rats were implanted with intravenous catheters, and the L5 and L6 spinal nerves were ligated in half of these animals. Rats were then trained to self-administer a commonly abused opioid (heroin) and commonly prescribed opioids (morphine, fentanyl, hydromorphone, and methadone). In addition, rats trained to self-administer heroin were given either clonidine or adenosine spinally before self-administration sessions to assess opioid-sparing effects. RESULTS: Nerve injury significantly decreased the reinforcing effects of low doses of opioids, and only doses of each opioid that reduced mechanical hypersensitivity maintained self-administration after spinal nerve ligation. The rate of drug consumption was correlated with the duration of the antiallodynic effect for each dose of opioid. Intrathecal administration of clonidine or adenosine reversed mechanical hypersensitivity, but only clonidine reduced heroin self-administration in rats with spinal nerve ligation. CONCLUSION: Opioid self-administration is significantly altered by nerve injury, with rate of drug intake being correlated with reversal of allodynia. Intrathecal clonidine, but not adenosine, produces opioid-sparing effects in self-administering rats. The neurobiologic mechanisms that regulate opioid consumption in rats therefore seem to be altered after nerve injury.     

The antiallodynic and antihyperalgesic effects of neurotropin in mice with spinal nerve ligation

Although Neurotropin(R) (NTP) has been used clinically as an analgesic in Japan for many years, its effect on neuropathic pain in animal models has not been examined in detail. Its main effect has been indicated to be activation of the descending monoaminergic pain inhibitory systems. To study the effect of NTP on neuropathic pain, we subjected mice to spinal nerve ligation. NTP inhibited both tactile allodynia and mechanical and thermal hyperalgesia in a dose-dependent manner. When the effect of NTP was examined after depletion of monoamines in the spinal cord by intrathecal neurotoxins, the antiallodynic and antihyperalgesic effects were still observed after serotonergic denervation, but not after noradrenergic denervation. In addition, intracerebroventricular NTP increased withdrawal threshold and latency although intrathecal or local administration of NTP did not. These results suggest that the antiallodynic and antihyperalgesic effect of NTP on neuropathic pain induced by spinal nerve ligation is mediated principally through the action at supraspinal sites and through activation of spinal noradrenergic systems, possibly via the descending inhibitory pathway.     

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.     

Antiallodynic effects of systemic and intrathecal morphine in the spared nerve injury model of neuropathic pain in rats

BACKGROUND: The efficacy of opioids for neuropathic pain remains controversial. The effects of morphine on pain behavior were investigated in two animal models of neuropathic pain: the spared nerve injury (SNI) model and the spinal nerve ligation (SNL) model. METHODS: Nerve injuries were created in rats either by tight ligation and section of the left tibial and common peroneal nerves (SNI) or by unilateral ligation of L5 and L6 spinal nerves (SNL). Paw withdrawal threshold to mechanical stimuli was measured using the up-down method in the hairy and glabrous skin territories of the sural nerve for SNI rats or in the mid-plantar paw of SNL rats. RESULTS: Before SNI, the median paw withdrawal thresholds in hairy and glabrous skin were similar (26 g [25%, 75% quartiles: 26, 26 g]). The paw withdrawal threshold decreased after SNI in both hairy and glabrous skin (P < 0.001). Thirty days after the SNI, the threshold in hairy skin (0.3 g) was significantly lower than in glabrous skin (1.9 g; P < 0.001). In blinded experiments, both subcutaneous and intrathecal morphine (0.1-10 microg) dose-dependently attenuated mechanical allodynia induced by SNI measured in the hairy skin, an effect that was naloxone reversible. The ED50 for the intrathecal morphine was 0.52 microg (95% confidence interval, 0.31-0.90 microg). Morphine (1 microg intrathecal) attenuated SNI-induced mechanical allodynia in glabrous skin with potency similar to that in hairy skin. In SNL rats, morphine (30 microg intrathecal) almost completely reversed the SNL-induced mechanical allodynia. CONCLUSIONS: (1) SNI-induced mechanical allodynia is characterized by a lower paw withdrawal threshold in hairy versus glabrous skin; (2) systemic and intrathecal morphine reverse SNI-induced mechanical allodynia in a dose-dependent fashion; and (3) intrathecal morphine also reverses SNL-induced mechanical allodynia. These results suggest that intrathecal opioids are likely to be effective in the treatment of neuropathic pain.     

Nerve Injury Induces a Tonic Bilateral μ-Opioid Receptor-mediated Inhibitory Effect on Mechanical Allodynia in Mice

Background: Mice lacking the [mu]-opioid receptor gene have been used to characterize the role of [mu]-opioid receptors in nociception and the analgesic actions of opioid agonists. In this study, the authors determined the role of [mu]-opioid receptors in neuropathic pain behaviors and the effectiveness of [mu]- and [kappa]-opioid receptor agonists on this behavior in mice.

Methods: The authors studied the behavioral responses of [mu]-opioid receptor knockout and wild-type mice to thermal and mechanical stimuli before and after neuropathic pain induced by unilateral ligation and section of the L5 spinal nerve. Response to mechanical stimuli was evaluated by determining the frequency of hind paw withdrawal to repetitive stimulation using a series of von Frey monofilaments. Thermal hyperalgesia was assessed by determining the paw withdrawal latencies to radiant heat and frequency of hind paw withdrawal to cooling stimuli. The effects of systemic morphine, the [kappa]-opioid agonist U50488H, and naloxone on responses to mechanical and thermal stimuli were also studied in spinal nerve-injured mice.

Results: After spinal nerve injury, wild-type mice developed increased responsiveness to mechanical, heat, and cooling stimuli ipsilateral to nerve injury. [mu]-Opioid receptor knockout mice not only had more prominent mechanical allodynia in the nerve-injured paw, but also expressed contralateral allodynia to mechanical stimuli. Hyperalgesia to thermal stimuli was similar between [mu]-opioid knockout and wild-type animals. Morphine decreased mechanical allodynia dose dependently (3-30 mg/kg subcutaneous) in wild-type mice-an effect that was attenuated in the heterozygous mice and absent in the homozygous [mu]-opioid knockout mice. The [kappa]-opioid agonist U50488H (3-10 mg/kg subcutaneous) attenuated mechanical allodynia in wild-type, heterozygous, and homozygous [mu]-opioid mice. Naloxone in wild-type mice resulted in enhanced ipsilateral and contralateral allodynia to mechanical stimuli that resembled the pain behavior observed in [mu]-opioid receptor knockout mice.     

The monoamine-mediated antiallodynic effects of intrathecally administered milnacipran, a serotonin noradrenaline reuptake inhibitor, in a rat model of neuropathic pain

Antidepressants are often used to treat neuropathic pain. In the present study, we determined the antiallodynic effects of selective monoamine reuptake inhibitors in the spinal cord in a rat model of neuropathic pain. Mechanical allodynia was produced by tight ligation of the left L5 and L6 spinal nerves and determined by applying von Frey filaments to the left hindpaw. A serotonin noradrenaline reuptake inhibitor, milnacipran, a selective serotonin reuptake inhibitor, paroxetine, or a selective noradrenaline reuptake inhibitor, maprotiline, was administered intrathecally via a chronically implanted catheter. Milnacipran produced dose-dependent antiallodynic effects at doses between 3 microg and 100 microg. The effect lasted for 7 h after injection of 100 microg (P < 0.05). The antiallodynic effect of 30 microg of milnacipran was attenuated by intrathecal coadministration of 30 microg of yohimbine, an alpha(2)-adrenoceptor antagonist, 30 microg of methysergide, a serotonin receptor antagonist, or 30 microg of atropine, a muscarinic receptor antagonist (P < 0.01, respectively). Intraperitoneal administration of milnacipran had no antiallodynic effects at doses of 3 to 30 mg/kg. Antiallodynic effects were not produced by intrathecal administration of paroxetine (10 to 100 microg) or maprotiline (10 to 100 microg). These findings suggest that simultaneous inhibition of serotonin and noradrenaline reuptake in the spinal cord is essential to mediate antiallodynic effects. Milnacipran might be effective for suppression of neuropathic pain.     

Effect of methylprednisolone on neuropathic pain and spinal glial activation in rats

BACKGROUND: Basic data are lacking regarding the efficacy and mechanisms of action of corticosteroids in neuropathic pain. Because recent studies indicate that spinal glial activation mediates the pathologic pain states, the authors sought to determine the effects of systemic and intrathecal methylprednisolone on the development and maintenance of neuropathic pain and spinal glial activation in a rat model. METHODS: Rats were anesthetized, and L5 and L6 spinal nerves were tightly ligated. Then, continuous infusion of systemic (4 mg x kg(-1) x day(-1)) or intrathecal (80 microg x kg(-1) x day(-1)) methylprednisolone or saline was started. Mechanical allodynia and thermal hyperalgesia were evaluated on days 4 and 7 postoperatively with von Frey and Hargreaves tests, respectively. Spinal astrocytic activation was evaluated with glial fibrillary acidic protein immunoreactivity on day 7. In other groups of rats, continuous 3-day treatment with intrathecal methylprednisolone or saline was started 7 days after spinal nerve ligation, when neuropathic pain had already developed. Behavioral tests and immunostaining were performed up to 3 weeks after the treatment. RESULTS: Spinal nerve ligation induced mechanical allodynia and thermal hyperalgesia on days 4 and 7 postoperatively. Glial fibrillary acidic protein immunoreactivity was remarkably enhanced on day 7. Both systemic and intrathecal methylprednisolone inhibited the development of neuropathic pain states and glial activation. Three-day treatment with intrathecal methylprednisolone reversed existing neuropathic pain state and glial activation up to 3 weeks after the treatment. CONCLUSION:: Systemic and intrathecal methylprednisolone inhibited spinal glial activation and the development and maintenance of a neuropathic pain state in a rat model of spinal nerve ligation.     

Effects of Radolmidine, A Novel α2-Adrenergic Agonist Compared with Dexmedetomidine in Different Pain Models in the Rat

Background: Intrathecally administered [alpha]2-adrenoceptor agonists produce effective antinociception, but sedation is an important adverse effect. Radolmidine is a novel [alpha]2-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 [mu]g, 2.5 [mu]g, 5 [mu]g, and 10 [mu]g.

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 [mu]g or 5 [mu]g 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 [mu]g 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.     

Role for cyclooxygenase 2 in the development and maintenance of neuropathic pain and spinal glial activation

BACKGROUND: Lines of evidence have indicated that cyclooxygenase 2 plays a role in the pathophysiology of neuropathic pain. However, the site and mechanism of its action are still unclear. Spinal glia has also been reported to mediate pathologic pain states. The authors evaluated the effect of continuous intrathecal or systemic cyclooxygenase-2 inhibitor on the development and maintenance of neuropathic pain and glial activation in a spinal nerve ligation model of rats. METHODS: Continuous intrathecal infusion of meloxicam (32 or 320 mug . kg . day) or saline was started immediately after L5-L6 spinal nerve ligation. Mechanical allodynia and thermal hyperalgesia were evaluated on days 4 and 7 postoperatively. Spinal astrocytic activation was evaluated with glial fibrially acidic protein immunoreactivity on day 7. In other groups of rats, continuous intrathecal meloxicam was started 7 days after spinal nerve ligation, and effects on established neuropathic pain and glial activation were evaluated. Last, effects of continuous systemic meloxicam (16 mg . kg . day) on existing neuropathic pain and glial activation were examined. RESULTS: Intrathecal meloxicam prevented the development of mechanical allodynia and thermal hyperalgesia induced by spinal nerve ligation. It also inhibited spinal glial activation responses. In contrast, when started 7 days after the nerve ligation, intrathecal meloxicam did not reverse established neuropathic pain and glial activation. Systemic meloxicam started 7 days after ligation partially reversed neuropathic behaviors but not glial activation. CONCLUSIONS: Spinal cyclooxygenase 2 mediates the development but not the maintenance of neuropathic pain and glial activation in rats. Peripheral cyclooxygenase 2 plays a part in the maintenance of neuropathic pain.     

Intrathecal lidocaine reverses tactile allodynia caused by nerve injuries and potentiates the antiallodynic effect of the COX inhibitor ketorolac

BACKGROUND: Systemic lidocaine and other local anesthetics reduce hypersensitivity states induced by both acute inflammation and peripheral nerve injury in animals and produce analgesia in some patients with chronic pain. The mechanisms underlying the antiallodynic effect of systemic lidocaine are unclear, although most focus is on peripheral mechanisms. Central mechanisms, particularly at the spinal dorsal horn level, are less known. In this study, the authors aimed to determine whether intrathecal lidocaine has an antiallodynic effect on established mechanical allodynia in two well-characterized neuropathic pain rat models: partial sciatic nerve ligation (PSNL) and spinal nerve ligation (SNL). METHODS: Lidocaine (100-300 micro g) was intrathecally injected in PSNL and SNL rats. The withdrawal threshold of both hind paws in response to mechanical stimulation was measured using a series of calibrated von Frey filaments. RESULTS: This single injection reduced ongoing tactile allodynia in PSNL and SNL rats. The antiallodynic effect of intrathecal lidocaine lasted longer in PSNL (> 3 days) than in SNL rats (< 3 days). Intraperitoneal lidocaine (300 micro g) had no effect on tactile allodynia in PSNL rats. In SNL rats, prior intrathecal lidocaine (200 and 300 micro g) potentiated the antiallodynic effect of intrathecal ketorolac, a nonselective cyclooxygenase inhibitor. Intrathecal ketorolac alone had no antiallodynic effect on SNL rats. However, prior intrathecal lidocaine (100 micro g) failed to potentiate the antiallodynic effect of intrathecal ketorolac. CONCLUSION: The authors' data suggest that intrathecal lidocaine possibly suppressed the hyperexcitability of the dorsal horn neurons and likely interacted with eicosanoid systems in the spinal dorsal horn.     

Intrathecal Lidocaine Reverses Tactile Allodynia Caused by Nerve Injuries and Potentiates the Antiallodynic Effect of the COX Inhibitor Ketorolac

Background: Systemic lidocaine and other local anesthetics reduce hypersensitivity states induced by both acute inflammation and peripheral nerve injury in animals and produce analgesia in some patients with chronic pain. The mechanisms underlying the antiallodynic effect of systemic lidocaine are unclear, although most focus is on peripheral mechanisms. Central mechanisms, particularly at the spinal dorsal horn level, are less known. In this study, the authors aimed to determine whether intrathecal lidocaine has an antiallodynic effect on established mechanical allodynia in two well-characterized neuropathic pain rat models: partial sciatic nerve ligation (PSNL) and spinal nerve ligation (SNL).

Methods: Lidocaine (100-300 [mu]g) was intrathecally injected in PSNL and SNL rats. The withdrawal threshold of both hind paws in response to mechanical stimulation was measured using a series of calibrated von Frey filaments.

Results: This single injection reduced ongoing tactile allodynia in PSNL and SNL rats. The antiallodynic effect of intrathecal lidocaine lasted longer in PSNL (> 3 days) than in SNL rats (< 3 days). Intraperitoneal lidocaine (300 [mu]g) had no effect on tactile allodynia in PSNL rats. In SNL rats, prior intrathecal lidocaine (200 and 300 [mu]g) potentiated the antiallodynic effect of intrathecal ketorolac, a nonselective cyclooxygenase inhibitor. Intrathecal ketorolac alone had no antiallodynic effect on SNL rats. However, prior intrathecal lidocaine (100 [mu]g) failed to potentiate the antiallodynic effect of intrathecal ketorolac.     

Allosteric adenosine modulation to reduce allodynia

BACKGROUND: Adenosine and adenosine agonists reduce hypersensitivity following inflammation and peripheral nerve injury models of chronic pain. Because inhibitors of adenosine reuptake or metabolism are also effective at reducing hypersensitivity, it is likely that there is a tonic release of spinal adenosine in these models. One approach to avoid adverse effects from direct agonists is to enhance the effect of the endogenous ligand by administering a positive allosteric modulator of its receptor. METHODS: Rats with mechanical hypersensitivity after spinal nerve ligation received intrathecal injections of adenosine, the allosteric adenosine receptor modulator T62, or their combination, or received systemic T62 alone or with intrathecal injection of a specific A1 adenosine antagonist. RESULTS: Both adenosine and T62 reduced hypersensitivity alone, with 50% maximal doses (ED50) of 14+/-5.9 and 3.7+/-0.8 microg, respectively. They interacted in an additive manner as determined by isobolography. T62 also reduced mechanical hypersensitivity after systemic administration (15 mg/kg), and this effect was blocked by intrathecal injection of 9 microg of the A1-specific adenosine receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine. CONCLUSIONS: These results add to previous studies that suggest ongoing spinal release of adenosine, which is antiallodynic, in this animal model of neuropathic pain. Positive allosteric modulation of the adenosine receptor reduces hypersensitivity by a spinal mechanism involving A1 adenosine receptor stimulation. Although obvious adverse effects were not observed in this investigation, further study is required to determine the feasibility of the use of such modulators in the treatment of chronic pain associated with hyperalgesia and allodynia.     

Spinal GABA(A) and GABA(B) receptor pharmacology in a rat model of neuropathic pain

BACKGROUND: This study tests the hypothesis that loss of spinal activity of gamma-aminobutyric acid (GABA) contributes to the allodynia and hyperalgesia observed after peripheral nerve injury. METHODS: Intrathecal catheters were implanted in male Sprague-Dawley rats. Antinociception was assessed by measuring withdrawal latency to immersion of the tail in a 52 degrees C water bath. Nerve injury was produced by ligation of the L5 and L6 spinal nerves. Testing was performed 4-14 days after spinal nerve ligation, when tactile allodynia and thermal hyperalgesia were established. Tactile allodynia was quantitated using the threshold to withdrawal of the hind paw on probing with von Frey filaments. Thermal hyperalgesia was quantitated using the latency to withdrawal of the hind paw from radiant heat. Motor function was tested using a rotarod apparatus. RESULTS: Spinal administration of the GABAA receptor antagonist bicuculline or the GABAB receptor antagonist phaclofen produced tactile allodynia and thermal hyperalgesia in normal rats. The GABAB receptor agonist baclofen, administered spinally, produced antinociception in the tail-flick test, whereas the GABAA receptor agonist isoguvacine did not. Isoguvacine and baclofen each reversed tactile allodynia and thermal hyperalgesia produced by spinal nerve ligation. Baclofen but not isoguvacine prolonged thermal withdrawal latency in nerve-injured rats beyond preoperative values. Baclofen but not isoguvacine impaired motor function. CONCLUSIONS: Pharmacologic inhibition of intrinsic GABA tone in normal rats resulted in tactile allodynia and thermal hyperalgesia, consistent with the hypothesis being tested. Exogenous administration of GABA agonists reversed spinal nerve ligation-induced allodynia and hyperalgesia, also consistent with this hypothesis. Isoguvacine produced specific antihyperalgesic and antiallodynic effects, whereas assessment of the effects of baclofen was complicated by motor dysfunction. Spinal GABAA agonists may provide a specific therapy for neuropathic pain.     

Characterization of the antinociceptive and pronociceptive effects of methadone in rats

BACKGROUND: Recently, it has been appreciated that in addition to their antinociceptive properties, opioid analgesics also can enhance pain sensitivity (opioid-induced hyperalgesia [OIH]). OIH may enhance preexisting pain and contribute to dose escalation, tolerance, and misuse/abuse of opioids. Better information is needed to determine which opioid or opioid combinations may be least likely to produce OIH and therefore possibly represent better choices for pain management. Herein the authors have examined the hyperalgesic and antinociceptive properties of racemic methadone and its enantiomers alone and in combination with morphine in rats. Methadone is of particular interest because it possesses both micro-receptor agonist and N-methyl-d-aspartate receptor antagonist activities. METHODS: The antinociceptive and hyperalgesic properties of d,l-methadone, l-methadone, and d-methadone were characterized by dose and sex using the thermal tail-flick test (high and low intensity). The responses to l- and d-methadone combinations with morphine were also determined with this model. RESULTS: Antinociceptive and hyperalgesic effects of d,l-methadone were demonstrated. These effects were related to dose but not to sex. The degree of hyperalgesia was greater with l-methadone compared with d,l-methadone. In contrast, d-methadone (N-methyl-d-aspartate antagonist) did not produce hyperalgesia. Furthermore, d-methadone blocked morphine hyperalgesia, enhanced antinociception, and abolished sex-related differences. This seems to be the result of antagonistic activity of d-methadone at the N-methyl-d-aspartate receptor. CONCLUSION: The current findings with methadone are supportive of previous findings implicating mu-opioid and N-methyl-d-aspartate receptor mechanisms in OIH. Better understanding of OIH may help in choosing the most appropriate opioids for use in the treatment of pain.     

Synergistic Antinociceptive Interactions of Morphine and Clonidine in Rats with Nerve-ligation Injury

Background: Ligation injury of the L5/L6 nerve roots in rats produces behavioral signs representative of clinical conditions of neuropathic pain, including tactile allodynia and thermal and mechanical hyperalgesia. In this model, intrathecal morphine shows no antiallodynic activity, as well as decreased antinociceptive potency and efficacy. This study was designed to explore the antinociceptive activity of intrathecal clonidine alone or in combination with intrathecal morphine (1:3 fixed ratio) in nerve-injured rats. The aims, with this study, were to use nerve-injured animals to determine: (1) whether the antinociceptive potency and efficacy of intrathecal clonidine was altered, and (2) whether the combination of intrathecal morphine and clonidine would act synergistically to produce antinociception.

Methods: Unilateral nerve injury was produced by ligation of the L5 and L6 spinal roots of male Sprague-Dawley rats. Sham-operated rats underwent a similar surgical procedure but without nerve ligation. Morphine and clonidine were given intrathecally through implanted catheters alone or in a 1:3 fixed ratio. Nociceptive responses were measured by recording tail withdrawal latency from a 55 degrees Celsius water bath, and data were calculated as % maximal possible effect (%MPE).

Results: Morphine produced a dose-dependent antinociceptive effect in both sham-operated and nerve-injured rats. The doses calculated to produce a 50 %MPE (i.e., A50) (+/- 95% confidence intervals [CI]) were 15 +/- 4.9 micro gram and 30 +/- 18 micro gram, respectively. Though morphine was able to produce a maximal response (100%) in sham-operated rats, the maximal response achieved in nerve-injured animals was only 69 +/- 21.9 %MPE. Clonidine produced a dose-dependent effect, with an A50 (+/- 95% CI) of 120 +/- 24 micro gram in sham-operated rats. In nerve-ligated rats, clonidine produced a maximal effect that reached a plateau of 55 +/- 10.9 %MPE and 49 +/- 10.2 %MPE at 100 and 200 micro gram, respectively, preventing the calculation of an A50. In sham-operated rats, a morphine-clonidine mixture produced maximal efficacy, with an A50 (+/- 95% CI) of 15 +/- 9.2 micro gram (total dose), significantly less than the theoretical additive A50 of 44 +/- 10 micro gram. In L5/L6 nerve-ligated rats, the morphine-clonidine combination produced maximal efficacy, with an A50 (+/- 95% CI) of 11 +/- 5.4 micro gram (total dose), which was significantly less than the theoretical additive A50 of 118 +/- 73 micro gram, indicating a synergistic antinociceptive interaction. The intrathecal injection of [D-Ala2, NMePhe4, Gly-ol]enkephalin (DAMGO) produced A50 values of 0.23 micro gram (range, 0.09-0.6) and 0.97 micro gram (range, 0.34-2.7) in sham-operated and ligated rats, respectively. Phentolamine (4 mg/kg, intraperitoneally) produced no antinociceptive effect alone and attenuated, rather than enhanced, the effect of morphine in both groups of rats.