Zemin Xu

Stereospecific Effect of Pregabalin on Ectopic Afferent Discharges and Neuropathic Pain Induced by Sciatic Nerve Ligation in Rats

Background The new anticonvulsants, gabapentin and pregabalin, are effective in the treatment of neuropathic pain. The sites and mechanisms of their analgesic action are not fully known. The authors have previously demonstrated that systemic gabapentin suppresses ectopic afferent discharges recorded from injured sciatic nerves in rats. In the current study, they further examined the stereospecific effect of pregabalin on neuropathic pain and afferent ectopic discharges in a rodent model of neuropathic pain. Methods Tactile allodynia and thermal hyperalgesia were induced by partial ligation of the left sciatic nerve in rats. Single-unit activity of afferent ectopic discharges was recorded from the sciatic nerve proximal to the site of ligation. Results Intravenous injection of 10–30 mg/kg pregabalin dose-dependently attenuated tactile allodynia (n = 10) and thermal hyperalgesia (n = 8). The stereoisomer of pregabalin, R-3-isobutylgaba, had no analgesic effect in this dose range. Furthermore, intravenous injection of pregabalin, but not R-3-isobutylgaba, significantly inhibited the ectopic discharges from injured afferents in a dose-dependent manner (from 20.8 ± 2.4 impulses/s during control to 2.3 ± 0.7 impulses/s after treatment with 30 mg/kg pregabalin, n = 15). Pregabalin did not affect the conduction velocity of afferent fibers and the response of normal afferent nerves to mechanical stimulation. Conclusions These data strongly suggest that the analgesic effect of pregabalin on neuropathic pain is likely mediated, at least in part, by its peripheral inhibitory action on the impulse generation of ectopic discharges caused by nerve injury.

Acetylcholine Stimulates the Release of Nitric Oxide from Rat Spinal Cord

Background Acetylcholine causes synthesis of nitric oxide in vascular endothelium, and presumptive evidence in vivo suggests spinally released acetylcholine causes antinociception and increased sympathetic nervous system activity via a nitric oxide mechanism. The purpose of this study was to determine, using a recently described bioassay system, whether acetylcholine stimulates nitric oxide release from spinal cord tissue in vitro. Methods Rat thoracolumbar spinal cord slices were incubated in a tissue chamber and perfused with Krebs-Henseleit solution. The perfusate was then passed through endothelium-denuded rat aortic rings and their tension was measured. Vascular rings were preconstricted with phenylephrine, then were exposed to spinal cord perfusate with increasing concentrations (10 sup -12 -10 sup -4 M) of acetylcholine alone or with various antagonists. Results Acetylcholine perfusion of spinal tissue caused concentration-dependent relaxations of the aortic rings, an effect blocked by each of the muscarinic antagonists, atropine, pirenzepine, and AFDX-116. Acetylcholine-induced relaxation also was antagonized by an inhibitor of nitric oxide synthase (N-methyl-L-arginine), a nitric oxide scavenger (hemoglobin) and an inhibitor of guanylate cyclase (methylene blue). Conclusions These results demonstrate release of a vasorelaxant from spinal cord tissue by acetylcholine, which results from an action on muscarinic receptors and exhibits a pharmacology consistent with nitric oxide. Although precise anatomic localization of acetylcholines action is not possible with this system, these results add to evidence that acetylcholine causes nitric oxide synthesis in the spinal cord.

Role of spinal muscarinic and nicotinic receptors in clonidine-induced nitric oxide release in a rat model of neuropathic pain.

Intrathecal administration of alpha(2) adrenergic agonists, such as clonidine, is capable of alleviating neuropathic pain. Recent studies suggest that spinal nitric oxide (NO) mediates the analgesic effect of intrathecal clonidine. Furthermore, compared to nicotinic receptors, spinal muscarinic receptors play a greater role in the analgesic effect of intrathecal clonidine. In the present study, we tested a hypothesis that clonidine-evoked NO release is dependent primarily on muscarinic receptors in the spinal cord after nerve injury. A rat model of neuropathic pain was induced by ligation of the left L(5)/L(6) spinal nerves. Using an in vitro spinal cord perfusion preparation, the effect of muscarinic and nicotinic receptor antagonists on clonidine-evoked nitrite (a stable product of NO) release was determined. Both muscarinic and nicotinic antagonists dose-dependently attenuated clonidine-elicited nitrite release. In spinal cords from the neuropathic rats, the inhibitory effect of muscarinic receptor antagonists (atropine and scopolamine) on clonidine-elicited nitrite release was more potent than that of nicotinic receptor antagonists (mecamylamine and hexamethonium). However, in spinal cords obtained from sham animals, the inhibitory effect of muscarinic and nicotinic antagonists did not differ significantly. These results indicate that muscarinic, as well as nicotinic, receptors mediate clonidine-induced NO release in the spinal cord. These data also suggest that after nerve injury, the cascade of activation of alpha(2) adrenergic receptors-muscarinic receptors-NO in the spinal cord likely plays a predominant role in the analgesic effect of intrathecal clonidine on neuropathic pain.

Location and Characteristics of Nitric Oxide Synthase in Sheep Spinal Cord and Its Interaction with α2-Adrenergic and Cholinergic Antinociception

Background Nitric oxide synthase is located in the spinal cord dorsal horn and intermediolateral cell column, where it may modulate sensory and sympathetic neuronal activity. However, the biochemical characteristics of this enzyme have not been examined in these different areas in the spinal cord. Although alpha2 -adrenergic agonists, muscarinic agonists, and nitric oxide may interact in the spinal cord to produce antinociception, these interactions have not been characterized. Methods Sheep spinal cord tissue was homogenized and centrifuged at high speed to separate soluble and membrane-bound fractions. Nitric oxide synthase activity was determined by conversion of [sup 14 Carbon]-L-arginine to [sup 14 Carbon]-L-citrulline and its kinetic characteristics, dependency on cofactors, and sensitivity to inhibitors determined. Sheep spinal cord was stained for nicotinamide adenine dinucleotide phosphate diaphorase as a marker for nitric oxide synthase. Antinociception to a mechanical stimulus from intrathecal clonidine alone and with neostigmine was determined and the effects of L-arginine and n-methyl-L-arginine were determined. Results More than 85% of nitric oxide synthase activity was present in the soluble form and its kinetic, cofactor, and antagonist properties were similar to those of the neuronal isoform of nitric oxide synthase. Biochemical and histochemical studies localized nitric oxide synthase to the superficial dorsal horn and the intermediolateral cell column. Clonidine antinociception was enhanced by L-arginine and neostigmine, but not by D-arginine. Neostigmines enhancement of clonidine antinociception was blocked by n-methyl-L-arginine, Conclusions These results confirm those of previous studies demonstrating localization of nitric oxide synthase to superficial dorsal horn and intermediolateral cell column of mammalian spinal cord, and suggesting its identity as the neuronal isoform. Spinal alpha2 -adrenergic agonist antinociception may be partly dependent on cholinergic and nitric oxide mechanisms.

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 &mgr;g, 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 &mgr;g 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.

Morphine-induced spinal cholinergic activation: in vivo imaging with positron emission tomography.

&NA; Positron emission tomography (PET) imaging of spinal cord in monkeys with a cholinergic tracer demonstrates increased spinal cholinergic activity in response to an analgesic dose of morphine, and this PET result correlates with measurement of acetylcholine spillover into spinal cord extracellular space induced by morphine, as measured by microdialysis. Previous studies in rats, mice, and sheep demonstrate activation of spinal cholinergic neurons by systemic opioid administration, and participation of this cholinergic activity in opioid‐induced analgesia. Testing the relevance of this observation in humans has been limited to measurement of acetylcholine spillover into lumbar cerebrospinal fluid. The purpose of this study was to apply a recently developed method to image spinal cholinergic terminals non‐invasively via PET and to test the hypothesis that the tracer utilized would reflect changes in local cholinergic activity. Following Animal Care and Use Committee approval, seven adult male rhesus monkeys were anesthetized on three separate occasions. On two of the occasions PET scans were performed using [18F] (+)‐4‐fluorobenzyltrozamicol ([18F]FBT), which selectively binds to the vesicular acetylcholine (ACh) transporter in the presynaptic cholinergic terminals. PET scans were preceded by injection of either saline or an analgesic dose of IV morphine (10 mg/kg). On the third occasion, microdialysis catheters were inserted in the spinal cord dorsal horn and acetylcholine concentrations in dialysates determined before and after IV morphine injection. Morphine increased cholinergic activity in the spinal cord, as determined by blood flow corrected distribution volume of [18F]FBT in the cervical cord compared to the cerebellum. Morphine also increased acetylcholine concentrations in microdialysates from the cervical cord dorsal horn. The one animal which did not show increased spinal cholinergic activity by PET from this dose of morphine also did not show increased acetylcholine from this morphine dose in the microdialysis experiment. These data confirm the ability to use PET to image spinal cholinergic terminals in the monkey spinal cord and suggest that acute changes in cholinergic activity can be imaged with this non‐invasive technique. Following preclinical screening, PET scanning with [18F]FBT may be useful to investigate mechanisms of analgesic action in normal humans and in those with pain.

Spinal neostigmine diminishes, but does not abolish, hypotension from spinal bupivacaine in sheep.

Spinal neostigmine causes analgesia in animals and humans and abolishes hypotension from spinal bupivacaine in rats.Since drug distribution and action can vary with the size of the spinal cord, we tested the effects of the maximum tolerated dose of spinal neostigmine alone and with bupivacaine in conscious sheep. Neostigmine alone increased arterial blood pressure by 10%, with a statistically significant increase beginning 30 min after injection. Compared with spinal bupivacaine alone, addition of neostigmine resulted in hypotension of slower onset (15 vs 5 min), shorter duration (45 vs 105 min), and smaller magnitude (-18% +/- 3% vs -37% +/- 6%). Addition of neostigmine did not affect height of sensory block from spinal bupivacaine. These data agree with preliminary clinical reports that spinal neostigmine diminishes, but does not abolish, hypotension from spinal bupivacaine in humans. (Anesth Analg 1996;83:1041-5)

Formation of 6-nitro-norepinephrine from nitric oxide and norepinephrine in the spinal cord and its role in spinal analgesia

Spinally released norepinephrine is thought to produce analgesia in part by stimulating alpha(2)-adrenergic receptors, which in turn leads to nitric oxide synthesis. Also, nitric oxide is known to react with norepinephrine in vivo in the brain to form 6-nitro-norepinephrine, which inhibits neuronal norepinephrine reuptake. In the present study, we tested the hypothesis that formation of 6-nitro-norepinephrine occurs in the spinal cord and that intrathecal administration of 6-nitro-norepinephrine produces analgesia by stimulating norepinephrine release. 6-Nitro-norepinephrine was present in rat spinal cord tissue and microdialysates of the dorsal horn and intrathecal space. Intrathecal norepinephrine injection increased 6-nitro-norepinephrine. 6-Nitro-norepinephrine also stimulated norepinephrine release in dorsal spinal cord in vitro. Intrathecal injection of 6-nitro-norepinephrine produced antinociception and interacted additively with norepinephrine for antinociception. Spinal noradrenergic nerve destruction increased antinociception from intrathecally injected norepinephrine, but decreased antinociception from 6-nitro-norepinephrine. These results suggest a functional interaction between spinal nitric oxide and norepinephrine in analgesia, mediated in part by formation of 6-nitro-norepinephrine. Stimulation of auto-inhibitory alpha(2)-adrenergic receptors at noradrenergic synapses decreases norepinephrine release. Paradoxically, alpha(2)-adrenergic agonist injection increases and alpha(2)-adrenergic antagonist injection decreases norepinephrine release in the spinal cord. 6-Nitro-norepinephrine may be an important regulator of spinal norepinephrine release and could explain the positive feedback on norepinephrine release after activation of spinal alpha(2)-adrenergic receptors.

In vivo imaging of the spinal cord cholinergic system with PET.

PURPOSE Our goal was to demonstrate the feasibility of an in vivo noninvasive method for imaging spinal cord cholinergic terminals using (+)-4-[18F]fluorobenzyltrozamicol ([18F]FBT) and PET. METHOD In vitro and in vivo experiments in rats were conducted to demonstrate the specific binding characteristics, localization, and time course of [3H]FBT binding in the spinal cord. PET imaging was then performed on seven rhesus monkeys. RESULTS The rat studies demonstrate high specific binding in the spinal cord with a distribution coinciding with the known distribution of cholinergic terminals. In vivo tracer concentrations in the spinal cord and basal ganglia were of the same magnitude. With use of [18F]FBT and PET in the rhesus monkey, the spinal cord was clearly visualized, with tracer concentration in the spinal cord being approximately one-fourth of that seen in the basal ganglia. CONCLUSION This work demonstrates the feasibility of imaging cholinergic terminals in vivo in the spinal cord using [18F]FBT and PET.