Masafumi Kimura

Dexmedetomidine decreases hyperalgesia in neuropathic pain by increasing acetylcholine in the spinal cord

The activation of α2-adrenoceptors has attracted attention as a therapeutic target for neuropathic pain, which remains a clinical challenge. In the present study, we examined the interaction between α2-adrenergic and cholinergic signaling in a rat model of neuropathic pain induced by spinal nerve ligation (SNL). Intrathecal administration of dexmedetomidine, which is a selective α2-adrenoceptor agonist (0.1-1.0 μg), dose-dependently suppressed hyperalgesia in SNL rats but did not alter paw withdrawal thresholds in normal rats. The analgesic effect of dexmedetomidine was abolished by intrathecal pretreatment with idazoxan (30 μg) and atropine (30 μg), which antagonize the α2-adrenoreceptor and muscarinic receptor, respectively. In vivo microdialysis in the lumbar spinal dorsal horn revealed that acetylcholine concentrations increased after dexmedetomidine perfusion (1 μM), but only in SNL rats. The combination of an ineffective dose of intrathecal dexmedetomidine with intraperitoneal donepezil, which is a cholinesterase inhibitor, decreased neuropathic hypersensitivity. These results suggest that plasticity of the spinal noradrenergic-cholinergic axis only occurs in neuropathic pain states. Thus, drug combinations that strengthen the noradrenergic-cholinergic interaction may provide therapeutic benefit in neuropathic pain.

Activation of astrocytes in the spinal cord contributes to the development of bilateral allodynia after peripheral nerve injury in rats.

Activation of spinal cord microglia and astrocytes after peripheral nerve injury contributes to the development of behavioral hypersensitivity. Suppression of spinal cord glial activation attenuates the development of nerve injury-induced allodynia. The contribution of spinal cord glia to existing allodynia, however, is not known. We investigated whether intrathecally administered propentofylline, a glial inhibitor, reverses existing allodynia after nerve injury. Male Sprague-Dawley rats underwent L5 spinal nerve transection, and mechanical allodynia was assessed by measuring hind paw withdrawal thresholds bilaterally using von Frey filaments. Rats received either saline or propentofylline (1, 3, and 10 μg/d) for 7 days (days 0-7) by intrathecal infusion with an osmotic minipump. Other groups of rats received either intrathecal infusion of saline or propentofylline (10 μg/d) for 7 days on days 14-21 or 60-67 after surgery. After completing the intrathecal infusion, lumbar spinal cord sections were assessed for immunostaining of astrocytic glial fibrillary acidic protein and microglial OX-42. Propentofylline infusion on days 0-7 suppressed development of allodynia in both the ipsilateral and contralateral hind paws in a dose-dependent manner. Propentofylline treatment on days 14-21 or 60-67 did not reverse existing allodynia. Propentofylline infusion (10 μg/d) inhibited astrocytic activation bilaterally on days 0-7, 14-21, and 60-67 and inhibited microglial activation on days 14-21 but not on days 0-7 and 60-67. These results suggest that activation of spinal glia, especially astrocytes, dominantly contributes to the development of neuropathic pain and also to mirror-image pain.

Monoamine-Dependent, Opioid-Independent Antihypersensitivity Effects of Intrathecally Administered Milnacipran, a Serotonin Noradrenaline Reuptake Inhibitor, in a Postoperative Pain Model in Rats

The neurotransmitters serotonin (5-HT) and noradrenaline (NA) have important roles in suppressing nociceptive transmission in the spinal cord. In the present study, we determined the efficacy and nature of the antihypersensitivity effects of milnacipran, a 5-HT and NA reuptake inhibitor (SNRI), in the spinal cord in a rat model of postoperative pain. Sprague-Dawley rats were used in all experiments. An incision was made on the plantar aspect of the hind paw. Mechanical hypersensitivity was measured by determining the withdrawal threshold to von Frey filaments applied to the paw. Drugs were administered intrathecally 24 h after paw incision. Microdialysis studies of the dorsal horn of the lumbar spinal cord were also performed to measure 5-HT and NA levels after systemic injection of milnacipran. Milnacipran (1–30 μg) produced dose-dependent antihypersensitivity effects. The effect lasted 6 h after the 30-μg injection. Doses of 30 μg or less produced no abnormal behavior. The peak antihypersensitivity effect of 10 μg of milnacipran was blocked by intrathecal pretreatment with antagonists of the α2-adrenoceptor (idazoxan; 30 μg) or 5-HT receptors (methysergide; 30 μg). Intrathecal pretreatment with 30 μg of naloxone, a μ-opioid receptor antagonist, did not reverse the effect of milnacipran. Isobolographic analysis indicated antinociceptive synergism between milnacipran and morphine. Microdialysis studies revealed that milnacipran increased both 5-HT and NA levels in the spinal dorsal horn. These findings suggest that the antihypersensitivity effect of intrathecal milnacipran in the postoperative pain model is monoamine-mediated. Combined administration of an SNRI with morphine might be a promising treatment to suppress postoperative hypersensitivity.

Supraspinal actions of nociceptin/orphanin FQ, morphine and substance P in regulating pain and itch in non‐human primates

Nociceptin/orphanin FQ (N/OFQ) peptide (NOP) receptor agonists display a promising analgesic profile in preclinical studies. However, supraspinal N/OFQ produced hyperalgesia in rodents and such effects have not been addressed in primates. Thus, the aim of this study was to investigate the effects of centrally administered ligands on regulating pain and itch in non‐human primates. In particular, nociceptive thresholds affected by intracisternal N/OFQ were compared with those of morphine and substance P, known to provide analgesia and mediate hyperalgesia, respectively, in humans.

Antihypersensitivity effects of tramadol hydrochloride in a rat model of postoperative pain.

BACKGROUND:Tramadol is used to treat a wide range of acute and chronic pain. This drug induces analgesia by 2 mechanisms of action: opioid receptor activation and enhancement of noradrenaline (NA) and serotonin (5-HT) transmission. The effect of tramadol on NA and 5-HT concentrations in the spinal cord, however, have not been assessed. In the present study, we investigated the antihypersensitivity effect of tramadol using a rat model of postoperative pain. We also evaluated the increase in NA and 5-HT levels in the spinal cord after tramadol injection using in vivo microdialysis. METHODS:We made a hindpaw incision in male Sprague-Dawley rats (postoperative pain model). Tramadol was administered intraperitoneally and intrathecally 24 hours after paw incision. Mechanical hypersensitivity was measured by determining the withdrawal threshold using von Frey filaments. Microdialysis studies from the dorsal horn of the lumbar spinal cord were performed to measure NA and 5-HT levels after intraperitoneal injection of tramadol. We also measured the NA and 5-HT content in the spinal cord in normal rats and rats with paw incision. RESULTS:Intraperitoneal (10, 20, and 40 mg/kg) and intrathecal (125, 250, and 500 &mgr;g) injection of tramadol produced an antihyperalgesic effect in a dose-dependent manner. The antihypersensitivity effect of tramadol was prevented by intrathecal pretreatment with methysergide (30 &mgr;g), a serotonin receptor antagonist; idazoxane (30 &mgr;g), a noradrenaline receptor antagonist; and naloxone (30 &mgr;g), a nonselective opioid receptor antagonist. Microdialysis study revealed that 5-HT and NA concentrations at the spinal dorsal horn were increased, peaking at 30 minutes after intraperitoneal injection of 20 mg/kg tramadol. Furthermore, the NA and 5-HT content in the ipsilateral dorsal half of the lumbar spinal cord was increased 1 day and 3 days after paw incision, respectively. CONCLUSIONS:These findings indicate that tramadol inhibits postoperative hypersensitivity by increasing NA and 5-HT levels in the spinal cord and activating opioid receptors. Tramadol might be more effective in the early postoperative period when spinal NA and 5-HT levels are increased.

Impaired Pain-evoked Analgesia after Nerve Injury in Rats Reflects Altered Glutamate Regulation in the Locus Coeruleus.

Background:Patients with neuropathic pain show reduced endogenous analgesia induced by a conditioned noxious stimulus. Here, the authors tested whether peripheral nerve injury impairs descending noradrenergic inhibition from the locus coeruleus (LC) after L5–L6 spinal nerve ligation (SNL) in rats. Methods:A subdermal injection of capsaicin was used to examine noxious stimulation–induced analgesia (NSIA), evoked LC glutamate and spinal noradrenaline release, and evoked LC neuronal activity in normal and SNL rats. The authors also examined the role of presynaptic metabotropic glutamate receptors or the astroglial glutamate transporter-1 (GLT-1). Results:SNL increased basal extracellular glutamate concentration in the LC (170.1%; 95% CI, 44.7 to 295.5; n = 15) and basal spinal cord noradrenaline release (252.1%; 95% CI, 113.6 to 391.3; n = 15), which was associated with an increased tonic LC neuronal activity and a down-regulation of GLT-1 in the LC. SNL reduced NSIA (−77.6%; 95% CI, −116.4 to −38.8; n = 14) and capsaicin evoked release of glutamate in the LC (−36.2%; 95% CI, −49.3 to −23.2; n = 8) and noradrenaline in the spinal cord (−38.8%; 95% CI, −45.1 to −32.5; n = 8). Capsaicin-evoked LC neuronal activation was masked in SNL rats. Removing autoinhibition of glutamatergic terminals by metabotropic glutamate receptor blockade or increasing GLT-1 expression by histone deacetylase inhibition restored NSIA in SNL rats. SNL-induced impairment of NSIA was mimicked in normal rats by knockdown of GLT-1 in the LC. Conclusions:These results suggest that increased extracellular glutamate in the LC consequent to down-regulation of GLT-1 contributes to LC dysfunction and impaired pain-evoked endogenous analgesia after nerve injury.

Ondansetron reverses antihypersensitivity from clonidine in rats after peripheral nerve injury: role of γ-aminobutyric acid in α2-adrenoceptor and 5-HT3 serotonin receptor analgesia.

Introduction: Monoaminergic pathways, impinging an &agr;2-adrenoceptors and 5-HT3 serotonin receptors, modulate nociceptive transmission, but their mechanisms and interactions after neuropathic injury are unknown. Here we examine these interactions in rodents after nerve injury. Methods: Male Sprague-Dawley rats following L5–L6 spinal nerve ligation (SNL) were used for either behavioral testing, in vivo microdialysis for &ggr;-aminobutyric acid (GABA) and acetylcholine release, or synaptosome preparation for GABA release. Results: Intrathecal administration of the &agr;2-adrenoceptor agonist (clonidine) and 5-HT3 receptor agonist (chlorophenylbiguanide) reduced hypersensitivity in SNL rats via GABA receptor-mediated mechanisms. Clonidine increased GABA and acetylcholine release in vivo in the spinal cord of SNL rats but not in normal rats. Clonidine-induced spinal GABA release in SNL rats was blocked by &agr;2-adrenergic and nicotinic cholinergic antagonists. The 5-HT3 receptor antagonist ondansetron decreased and chlorophenylbiguanide increased spinal GABA release in both normal and SNL rats. In synaptosomes from the spinal dorsal horn of SNL rats, presynaptic GABA release was increased by nicotinic agonists and decreased by muscarinic and &agr;2-adrenergic agonists. Spinally administered ondansetron significantly reduced clonidine-induced antihypersensitivity and spinal GABA release in SNL rats. Conclusion: These results suggest that spinal GABA contributes to antihypersensitivity from intrathecal &agr;2-adrenergic and 5-HT3 receptor agonists in the neuropathic pain state, that cholinergic neuroplasticity after nerve injury is critical for &agr;2-adrenoceptor-mediated GABA release, and that blockade of spinal 5-HT3 receptors reduces &agr;2-adrenoceptor-mediated antihypersensitivity via reducing total GABA release.

Relief of Hypersensitivity after Nerve Injury from Systemic Donepezil Involves Spinal Cholinergic and γ-Aminobutyric Acid Mechanisms

Background:Evoking spinal release of acetylcholine (ACh) produces antinociception in normal animals and reduces hypersensitivity after nerve injury, and some studies suggest that ACh-mediated analgesia relies on &ggr;-aminobutyric acid (GABA)-ergic signaling in the spinal cord. In this study, the authors tested the spinal mechanisms underlying the antihypersensitivity effects of donepezil, a central nervous system–penetrating cholinesterase inhibitor, in a rat model of neuropathic pain. Methods:Male Sprague-Dawley rats were anesthetized, and L5 spinal nerve ligation was performed unilaterally. Withdrawal threshold to a paw pressure test was measured before and after intraperitoneal administration of donepezil, with or without intrathecal antagonists for cholinergic and GABAergic receptors. Microdialysis studies in the ipsilateral dorsal horn of the lumbar spinal cord were also performed to measure extracellular ACh and GABA. Results:Donepezil increased the withdrawal threshold in spinal nerve ligation rats but not in normal rats. The antihypersensitivity effect of donepezil (1 mg/kg) in spinal nerve ligation rats was reduced by intrathecal pretreatment with atropine (30 &mgr;g), a muscarinic receptor antagonist; mecamylamine (100 &mgr;g), a nicotinic receptor antagonist; bicuculline (0.03 &mgr;g), a &ggr;-aminobutyric acid receptor type A antagonist; and CGP 35348 (30 &mgr;g), a &ggr;-aminobutyric acid receptor type B antagonist. ACh and GABA concentrations in the microdialysates from the spinal dorsal horn were increased after intraperitoneal donepezil treatment (1 mg/kg) in both normal and spinal nerve ligation rats. Conclusions:Systemic administration of donepezil reduces hypersensitivity after nerve injury by increasing extracellular ACh concentration, which itself induces GABA release in the spinal cord. Activation of this spinal cholinergic–GABAergic interaction represents a promising treatment for neuropathic pain.

Effects of cardiopulmonary resuscitation at high altitudes on the physical condition of untrained and unacclimatized rescuers.

OBJECTIVE The authors experienced a case of prolonged cardiopulmonary resuscitation (CPR) on Mount Fuji (3776 m) that demanded strenuous work by the rescuers. The objective of this study was to provide information regarding the physiologic effects on the rescuers of performing CPR at moderate altitude. METHODS The effects of CPR at 2700 m and 3700 m above sea level on the physical condition of the rescuers were studied in 8 male volunteers. RESULTS Cardiopulmonary resuscitation for 5 minutes at 3700 m significantly reduced arterial blood oxygen saturation and increased rate-pressure products (P < .05). Scores on the Borg scale, a subjective score of fatigue, after CPR action at 2700 m (P < .05) and 3700 m (P < .01) were higher than the scores at sea level. CONCLUSIONS Prolonged CPR at high altitude exerts a significant physical effect upon the condition of rescuers. A role for mechanical devices should be considered wherever possible.

Gabapentin loses efficacy over time after nerve injury in rats: Role of glutamate transporter-1 in the locus coeruleus.

Abstract Despite being one of the first-choice analgesics for chronic neuropathic pain, gabapentin sometimes fails to provide analgesia, but the mechanisms for this lack of efficacy is unclear. Rats with nerve injury including L5-L6 spinal nerve ligation (SNL) respond uniformly and well to gabapentin, but many of these studies are performed within just a few weeks of injury, questioning their relevance to chronic neuropathic pain. In this study, intraperitoneal gabapentin showed a time-dependently reduction in antihypersensitivity after SNL, associated with downregulation of astroglial glutamate transporter-1 (GLT-1) in the locus coeruleus (LC). Consistently, SNL also time-dependently increased basal but masked gabapentin-induced noradrenergic neuronal activity in the LC. In rats 2 weeks after SNL, knock-down of GLT-1 in the LC reduced the antihypersensitivity effect of gabapentin. In rats 8 weeks after SNL, increasing GLT-1 expression by histone deacetylase inhibitor valproate restored the antihypersensitivity effect of gabapentin, associated with restored gabapentin-induced noradrenergic neuronal activity in the LC and subsequent spinal noradrenaline release. Knock-down of GLT-1 in the LC reversed the effect of valproate to restore gabapentin-induced antihypersensitivity. In addition, the antihypersensitivity effect of the intrathecal &agr;2-adrenoceptor agonist clonidine also decreased with time after SNL injury. These results suggest that downregulation of GLT-1 in the LC and reduced spinal noradrenergic inhibition contribute to impaired analgesic efficacy from gabapentin in chronic neuropathic pain and that valproate can rescue this impaired efficacy.