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.     

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 [mu]g [middle dot] kg-1 [middle dot] day-1) 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 [middle dot] kg-1 [middle dot] day-1) 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.     

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.     

Synergistic effect between intrathecal non-NMDA antagonist and gabapentin on allodynia induced by spinal nerve ligation in rats

BACKGROUND: Glutamate and non-N-methyl-D-aspartate (NMDA) receptors have been implicated in the development of neuroplasticity in the spinal cord in neuropathic pain. The spinal cord has been identified as one of the sites of the analgesic action of gabapentin. In the current study, the authors determined the antiallodynic effect of intrathecal 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) in a rat model of neuropathic pain. Also tested was a hypothesis that intrathecal injection of CNQX and gabapentin produces a synergistic effect on allodynia in neuropathic rats. METHODS: Allodynia was produced in rats by ligation of the left L5 and L6 spinal nerves. Allodynia was determined by application of von Frey filaments to the left hind paw. Through an implanted intrathecal catheter, 10-100 microg gabapentin or 0.5-8 microg CNQX disodium (a water-soluble formulation of CNQX) was injected in conscious rats. Isobolographic analysis was performed comparing the interaction of intrathecal gabapentin and CNQX using the ED50 dose ratio of 15:1. RESULTS: Intrathecal treatment with gabapentin or CNQX produced a dose-dependent increase in the withdrawal threshold to mechanical stimulation. The ED50 for gabapentin and CNQX was 45.9+/-4.65 and 3.4+/-0.22 microg, respectively. Intrathecal injection of a combination of CNQX and gabapentin produced a strong synergistic antiallodynic effect in neuropathic rats. CONCLUSIONS: This study shows that intrathecal administration of CNQX exhibits an antiallodynic effect in this rat model of neuropathic pain. Furthermore, CNQX and gabapentin, when combined intrathecally, produce a potent synergistic antiallodynic effect on neuropathic pain in spinal nerve-ligated rats.     

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.     

Antiallodynic Effect of Intrathecal Neostigmine Is Mediated by Spinal Nitric Oxide in a Rat Model of Diabetic Neuropathic Pain

Background: Intrathecal administration of acetylcholinesterase inhibitors produces antinociception in both animals and humans, but their effect on diabetic neuropathic pain has not been studied. In the current study, we determined the antiallodynic effect of intrathecal injection of an acetylcholinesterase inhibitor, neostigmine, in a rat model of diabetic neuropathic pain. In addition, since acetylcholine can increase release of nitric oxide in the spinal cord, we studied the role of spinal endogenous nitric oxide in the action of intrathecal neostigmine in diabetic neuropathic pain.

Methods: Rats were rendered diabetic with an intraperitoneal 50-mg/kg injection of streptozotocin. Intrathecal catheters were inserted, with tips in the lumbar intrathecal space. Mechanical allodynia was determined by application of von Frey filaments to the hind paw. We first determined the dose-dependent effect of intrathecal neostigmine on allodynia. The role of spinal nitric oxide in the action of intrathecal neostigmine was then examined through intrathecal treatments with a neuronal nitric oxide synthase inhibitor (TRIM), a nitric oxide scavenger (PTIO), l-arginine, or d-arginine.

Results: The diabetic rats developed a sustained tactile allodynia within 4 weeks after streptozotocin injection. Intrathecal injection of 0.1-0.5 [mu]g neostigmine dose-dependently increased the withdrawal threshold in response to application of von Frey filaments. Intrathecal pretreatment with 30 [mu]g TRIM or 30 [mu]g PTIO abolished the antiallodynic effect of intrathecal neostigmine. Furthermore, the inhibitory effect of TRIM on the action of intrathecal neostigmine was reversed by intrathecal injection of 100 [mu]g l-arginine but not d-arginine.     

Sex differences in cholinergic analgesia II: differing mechanisms in two models of allodynia.

BACKGROUND: Cholinergic agents reduce allodynia after nerve injury in animals and may be useful in the treatment of neuropathic pain. Intrathecally administered neostigmine and neuronal nicotinic agonists are more potent in female than in male rats against acute thermal noxious stimuli. The purpose of this study was to determine whether there is also a sex difference in the antiallodynic effects of intrathecal cholinomimetic agents in two models of allodynia and to test their pharmacologic mechanisms. METHODS: Male and female rats with indwelling intrathecal catheters received injections of neostigmine, bethanechol (muscarinic agonist), RJR-2403 (neuronal nicotinic agonist) alone or with atropine (muscarinic antagonist), mecamylamine (nicotinic antagonist), phentolamine (alpha-adrenergic antagonist), or saline control. The effect of these agents was determined on mechanical allodynia produced by either intraplantar injection of capsaicin or ligation of spinal nerves. RESULTS: Neostigmine and RJR-2403 but not bethanechol were more potent in female than in male rats in reducing allodynia after nerve injury, and antagonist studies were also consistent with a nicotinic component to explain this sex difference. Phentolamine did not reverse neostigmine's effect. In contrast, for capsaicin-induced allodynia, neostigmine plus mecamylamine but not neostigmine or RJR-2403 was more potent in female than in male rats. CONCLUSIONS: These data demonstrate a sex difference of intrathecal neostigmine after nerve injury-induced allodynia similar to that observed in normal animals that received acute noxious thermal stimulation. However, this sex difference is not universal to all pain models because it was not present after intradermal capsaicin injection, nor is its interaction with spinal noradrenergic mechanisms consistent in all models.     

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.     

Synergistic Effect between Intrathecal Non-NMDA Antagonist and Gabapentin on Allodynia Induced by Spinal Nerve Ligation in Rats

Background: Glutamate and non-N-methyl-D-aspartate (NMDA) receptors have been implicated in the development of neuroplasticity in the spinal cord in neuropathic pain. The spinal cord has been identified as one of the sites of the analgesic action of gabapentin. In the current study, the authors determined the antiallodynic effect of intrathecal 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) in a rat model of neuropathic pain. Also tested was a hypothesis that intrathecal injection of CNQX and gabapentin produces a synergistic effect on allodynia in neuropathic rats.

Methods: Allodynia was produced in rats by ligation of the left L5 and L6 spinal nerves. Allodynia was determined by application of von Frey filaments to the left hind paw. Through an implanted intrathecal catheter, 10-100 [mu]g gabapentin or 0.5-8 [mu]g CNQX disodium (a water-soluble formulation of CNQX) was injected in conscious rats. Isobolographic analysis was performed comparing the interaction of intrathecal gabapentin and CNQX using the ED50 dose ratio of 15:1.

Results: Intrathecal treatment with gabapentin or CNQX produced a dose-dependent increase in the withdrawal threshold to mechanical stimulation. The ED50 for gabapentin and CNQX was 45.9 +/- 4.65 and 3.4 +/- 0.22 [mu]g, respectively. Intrathecal injection of a combination of CNQX and gabapentin produced a strong synergistic antiallodynic effect in neuropathic rats.     

Role of Spinal NO in Antiallodynic Effect of Intrathecal Clonidine in Neuropathic Rats

Background: The role of spinal nitric oxide (NO) in neuropathic pain remains uncertain. Although intrathecal clonidine causes NO release in the spinal cord, the functional role of spinal No in clonidine-produced analgesia has not been examined. The objectives of this study were to assess the role of spinal NO in maintenance of allodynia and to determine the role of spinal NO in the antiallodynic effect of intrathecal clonidine.

Methods: Allodynia was produced in rats by tight ligation of the left L5-L6 spinal nerves. Intrathecal catheters were inserted with tips in the lumbar intrathecal space. Mechanical allodynia was determined by application of von Frey filaments to the left hindpaw. In the first series of experiments, allodynia was assessed before and after intrathecal injection of saline, L-arginine, an NO donor (SNAP), two NO synthase inhibitors (TRIM and NMMA), or an NO scavenger (PTIO). In the second series of experiments, 20 [micro sign]g of clonidine was injected intrathecally 15 min after intrathecal injection of saline, TRIM, NMMA, or PTIO.

Results: Allodynia was not affected significantly by intrathecal injection of L-arginine, SNAP, TRIM, NMMA, or PTIO. The antiallodynic effect produced by intrathecal injection of clonidine was attenuated significantly by pretreatment with TRIM, NMMA, or PTIO.     

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 [middle dot] kg-1 [middle dot] day-1) or intrathecal (80 [mu]g [middle dot] kg-1 [middle dot] 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.     

Antiallodynic effect of intrathecal neostigmine is mediated by spinal nitric oxide in a rat model of diabetic neuropathic pain

BACKGROUND: Intrathecal administration of acetylcholinesterase inhibitors produces antinociception in both animals and humans, but their effect on diabetic neuropathic pain has not been studied. In the current study, we determined the antiallodynic effect of intrathecal injection of an acetylcholinesterase inhibitor, neostigmine, in a rat model of diabetic neuropathic pain. In addition, since acetylcholine can increase release of nitric oxide in the spinal cord, we studied the role of spinal endogenous nitric oxide in the action of intrathecal neostigmine in diabetic neuropathic pain. METHODS: Rats were rendered diabetic with an intraperitoneal 50-mg/kg injection of streptozotocin. Intrathecal catheters were inserted, with tips in the lumbar intrathecal space. Mechanical allodynia was determined by application of von Frey filaments to the hind paw. We first determined the dose-dependent effect of intrathecal neostigmine on allodynia. The role of spinal nitric oxide in the action of intrathecal neostigmine was then examined through intrathecal treatments with a neuronal nitric oxide synthase inhibitor (TRIM), a nitric oxide scavenger (PTIO), L-arginine, or D-arginine. RESULTS: The diabetic rats developed a sustained tactile allodynia within 4 weeks after streptozotocin injection. Intrathecal injection of 0.1-0.5 microg neostigmine dose-dependently increased the withdrawal threshold in response to application of von Frey filaments. Intrathecal pretreatment with 30 microg TRIM or 30 microg PTIO abolished the antiallodynic effect of intrathecal neostigmine. Furthermore, the inhibitory effect of TRIM on the action of intrathecal neostigmine was reversed by intrathecal injection of 100 microg L-arginine but not D-arginine. CONCLUSIONS: Intrathecal neostigmine produces a profound analgesic effect in a rat model of diabetic neuropathic pain. Spinal endogenous nitric oxide contributes to the analgesic action of intrathecal neostigmine in this rat model of diabetic neuropathic pain.     

Morphine, oxycodone, methadone and its enantiomers in different models of nociception in the rat

We studied the effects of the commonly used mu-opioid receptor agonists morphine, oxycodone, methadone and the enantiomers of methadone in thermal and mechanical models of acute pain and in the spinal nerve ligation model of neuropathic pain in rats. Subcutaneous administration of morphine, oxycodone, and methadone produced a dose-dependent antinociceptive effect in the tail flick, hotplate, and paw pressure tests. l-methadone, racemic methadone, and oxycodone had a similar dose-dependent antinociceptive effect, whereas the dose-response curve of morphine was shallower. In the spinal nerve ligation model of neuropathic pain, subcutaneous administration of morphine, oxycodone, methadone and l-methadone had antiallodynic effects in tests of mechanical and cold allodynia. l-methadone showed the strongest antiallodynic effect of the tested drugs. d-methadone was inactive in all tests. Morphine 5.0 mg/kg, oxycodone 2.5 mg/kg, and l-methadone 1.25 mg/kg decreased spontaneous locomotion 30 min after drug administration. In conclusion, in acute nociception all mu-opioid receptor agonists produced antinociception, with morphine showing the weakest effect. In nerve injury pain, l-methadone showed the greatest antiallodynic potency in both mechanical and cold allodynia compared with the other opioids. Opioids seem to have different profiles in different pain models. l-methadone should be studied for neuropathic pain in humans.     

Intrathecal Clonidine Alleviates Allodynia in Neuropathic Rats: Interaction with Spinal Muscarinic and Nicotinic Receptors

Background: Intrathecally administered clonidine increases release of spinal acetylcholine, which may be related to its analgesic action in neuropathic pain. The current study determined the role of spinal muscarinic and nicotinic receptors in the antiallodynic effect of intrathecally administered clonidine in spinal nerve-ligated rats.

Methods: Allodynia was produced in rats by ligation of the left L5-L6 spinal nerves. Mechanical allodynia was determined by application of von Frey filaments to the left hindpaw. The effect of intrathecal injection of saline, two muscarinic receptor antagonists (atropine and scopolamine), and two nicotinic receptor antagonists (mecamylamine and hexamethonium) on the antiallodynic action produced by intrathecal administration of 20 [micro sign]g clonidine was assessed in six groups of animals. Each group consisted of six to eight rats.

Results: Intrathecal injection of saline or muscarinic or nicotinic receptor antagonists did not alter the withdrawal thresholds. The antiallodynic effect produced by intrathecally administered clonidine was attenuated in a dose-dependent manner by intrathecal treatment with muscarinic and nicotinic antagonists. Although nicotinic receptor antagonists only partially attenuated the effect of clonidine, blockade of spinal muscarinic receptors almost abolished the antiallodynic effect of clonidine.     

Role of the neurosteroid allopregnanolone in the hyperalgesic behavior induced by painful nerve injury in rats

The neurosteroid allopregnanolone (AP) influences the excitability of the central nervous system by acting as a positive allosteric modulator of γ-aminobutyric acid type A (GABA_A) receptors. Here, we investigated the role of AP and its therapeutic potential in rats that showed hyperalgesic behavior after undergoing spinal nerve ligation (SNL). AP levels measured in the spinal cord and brain of rats that underwent SNL were greater than the corresponding levels in control animals. More importantly, spinal AP levels in hyperalgesic rats were lower than those in the rats that did not develop hyperalgesia following SNL; in contrast, brain AP levels were comparable among these groups. No differences in serum AP levels were observed among the groups. In addition, intrathecal exogenous administration of AP showed the antihyperalgesic effects in hyperalgesic rats after SNL. These findings suggest that changes in spinal AP biosynthesis are involved in the pathogenesis of neuropathic pain following peripheral nerve injury, and pharmacological manipulation of this phenomenon may provide a potential therapeutic target for neuropathic pain.     

Antihyperalgesic and side effects of intrathecal clonidine and tizanidine in a rat model of neuropathic pain

BACKGROUND: Although intrathecal clonidine produces pronounced analgesia, antinociceptive doses of intrathecal clonidine produce several side effects, including hypotension, bradycardia, and sedation. Intrathecal tizanidine, another alpha(2)-adrenergic agonist, has provided antinociception without producing pronounced hemodynamic changes in animal studies. However, it has been unclear whether antihyperalgesic doses of intrathecal clonidine and tizanidine produce hypotension and bradycardia in a neuropathic pain state. This study was designed to evaluate the antihyperalgesic effects and side effects of intrathecal clonidine and tizanidine in a rat model of neuropathic pain. METHODS: Male Sprague-Dawley rats were chronically implanted with lumbar intrathecal catheters, and the sciatic nerve was loosely ligated. After 21-28 days after surgery, the rats received intrathecal clonidine (0.3, 1.0, and 3.0 microg) and tizanidine (1.0, 2.0, and 5.0 microg), and the antihyperalgesic effects of thermal and mechanical stimuli were examined. In addition, the changes in blood pressure and heart rate, sedation level, and other side effects after intrathecal administration of drugs were recorded. RESULTS: The administration of 3.0 microg intrathecal clonidine or 5.0 microg tizanidine significantly reversed both thermal and mechanical hyperalgesia. The administration of 3.0 microg intrathecal clonidine, but not 5.0 microg tizanidine, significantly decreased mean blood pressure and heart rate and produced urinary voiding. A greater sedative effect was produced by 3.0 microg intrathecal clonidine than by 5.0 microg tizanidine. CONCLUSION: The antihyperalgesic dose of intrathecal clonidine and the antinociceptive doses produced several side effects. Intrathecal tizanidine at the dose that reversed hyperalgesia would be preferable for neuropathic pain management because of absence of hypotension and bradycardia and lower incidence of sedation.     

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.     

Electrophysiologic evidence for increased endogenous gabaergic but not glycinergic inhibitory tone in the rat spinal nerve ligation model of neuropathy

BACKGROUND: Changes in the inhibitory activity mediated by gamma-aminobutyric acid (GABA) and glycine, acting at spinal GABAA receptors and strychnine-sensitive glycine receptors, are of interest in the development of neuropathic pain. There is anatomic evidence for changes in these transmitter systems after nerve injuries, and blocking either GABAA or glycine receptors has been shown to produce allodynia-like behavior in awake normal animals. METHODS: In this study, the possible changes in GABAergic and glycinergic inhibitory activity in the spinal nerve ligation model of neuropathic pain were studied by comparing the effects of the GABAA-receptor antagonist bicuculline and the glycine-receptor antagonist strychnine in neuropathic rats to their effects in sham-operated and nonoperated control rats. RESULTS: Bicuculline produced a dose-related facilitation of the Adelta-fiber-evoked activity in all study groups and increased C-fiber-mediated activity in the spinal nerve ligation group but not in either of the control groups. There were no differences in the effect of bicuculline on low threshold responses between the study groups. The glycine receptor antagonist strychnine did not have a statistically significant effect on any of the parameters studied in any of the control groups. CONCLUSIONS: These results support the idea of an increased GABAergic inhibitory tone in the spinal cord of neuropathic rats, possibly as compensation for increased excitability after nerve injury.     

Intrathecal adenosine interacts with a spinal noradrenergic system to produce antinociception in nerve-injured rats.

BACKGROUND: Adenosine analogs produce antinociception in animal models of acute pain, reduce hypersensitivity in models of inflammatory and nerve-injury pain, and stimulate neurotransmitter release in the brain. Adenosine itself is entering clinical trials for analgesia, and the current study examined the effect, mechanisms of action, and interaction with noradrenergic systems of intrathecal adenosine in a rat model of neuropathic pain. METHODS: The left L5 and L6 spinal nerve roots were ligated and, 1 week later, an intrathecal catheter was inserted in male rats. Withdrawal threshold to mechanical stimulation of the left hind paw was determined before and after surgery, confirming mechanical hypersensitivity. The effects of intrathecal adenosine, clonidine, and their combination on withdrawal threshold were determined, and reversal of the effects of adenosine by adenosine and alpha2-adrenergic antagonists and by destruction of noradrenergic nerve terminals was tested. Finally, spinal cord slices were perfused in vitro with the adenosine agonist 5'-N-ethylcarboxamide adenosine, and norepinephrine release was measured. RESULTS: Intrathecal adenosine and clonidine reduced hypersensitivity and interacted in an additive manner. The effects of adenosine were blocked by intrathecal injection of A1 but not A2 adenosine receptor antagonists, by an alpha2-adrenergic antagonist, and by destruction of spinal noradrenergic nerve terminals. Perfusion of spinal cord slices with 5'-N-ethylcarboxamide adenosine resulted in a concentration-dependent increase in norepinephrine release. CONCLUSION: These data support clinical examination of intrathecal adenosine alone and with clonidine in the treatment of chronic pain states that include a component of mechanical hypersensitivity and suggest that, after nerve injury, adenosine acts to reduce hypersensitivity through spinal norepinephrine release.