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Pain Medicine  |   May 2002
Spinal GABAAand GABABReceptor Pharmacology in a Rat Model of Neuropathic Pain
Author Affiliations & Notes
  • T. Philip Malan, Ph.D., M.D.
    *
  • Heriberto P. Mata, B.S.
  • Frank Porreca, Ph.D.
    *
  • *Professor, Departments of Anesthesiology and Pharmacology, †Research Specialist, Department of Anesthesiology.
  • Received from the Departments of Anesthesiology and Pharmacology, The University of Arizona, Tucson, Arizona.
Article Information
Pain Medicine
Pain Medicine   |   May 2002
Spinal GABAAand GABABReceptor Pharmacology in a Rat Model of Neuropathic Pain
Anesthesiology 5 2002, Vol.96, 1161-1167. doi:
Anesthesiology 5 2002, Vol.96, 1161-1167. doi:
IT has been hypothesized that one of the factors underlying the enhanced pain sensitivity observed in neuropathic pain states is the loss of activity of the inhibitory neurotransmitter γ-aminobutyric acid (GABA) in the spinal cord. 1,2 If enhanced pain sensitivity is produced by loss of spinal GABAergic tone, spinal administration of GABA agonists might be predicted to be effective against neuropathy-induced allodynia and hyperalgesia.
The current study tests the hypothesis that loss of spinal GABA tone contributes to the allodynia and hyperalgesia observed after nerve injury by examining whether blockade of spinal GABAAand GABABreceptors is sufficient to lower tactile and thermal sensory thresholds. Yaksh 3 reported that intrathecal administration of the GABAAreceptor–selective antagonist bicuculline resulted in increased responsiveness to tactile stimuli, demonstrating that loss of activity at the GABAAreceptor is sufficient to produce tactile allodynia. Response to thermal stimuli was not tested. Spinal bicuculline was also shown to produce hypersensitivity of dorsal horn neurons, again suggesting the presence of a chronic inhibitory GABA tone in spinal cord. 4–7 Administration of the GABABreceptor antagonist CGP35348 resulted in pain-like responses to innocuous mechanical stimulation, demonstrating that loss of activity at the GABABreceptor is sufficient to produce tactile allodynia. 8 With the data presented in this article, we confirmed that blockade of GABAAor GABABreceptor tone produces tactile allodynia. We also tested the hypothesis that loss of GABAAor GABABreceptor tone increases sensitivity to thermal stimuli. Study of responses to both mechanical and thermal stimuli is important because thermal hyperalgesia has different spinal pharmacology and neural circuitry than does tactile allodynia. 9–11 
Ligation of the L5 and L6 spinal nerves in rats produces a model of neuropathic pain that results in tactile allodynia, as manifested by increased sensitivity to probing of the hind paw with von Frey filaments, and thermal hyperalgesia, as manifested by decreased latency to withdrawal from a radiant heat source. 12,13 In the spinal nerve ligation (SNL) model, Hwang and Yaksh 14 demonstrated that allodynia was reversed by the GABAAreceptor agonist muscimol and the GABABreceptor agonist baclofen. They also found that muscimol and baclofen resulted in dose-dependent hind limb motor weakness at doses greater than those required for antagonism of allodynia. Other studies using other models of neuropathic pain have also found GABA receptor agonists to reverse mechanical hyperalgesia. 15,16 Systemic administration of midazolam, which facilitates the action of GABA at the GABAAreceptor, reduced A-δ– and C-fiber–evoked activity in dorsal horn neurons. 17 In the current experiments, we confirmed the effectiveness of intrathecal GABAAand GABABreceptor agonists in reversing tactile allodynia produced by SNL. In addition, we tested the hypothesis that GABAAand GABABreceptor agonists also reverse nerve injury–induced thermal hyperalgesia. Finally, we examined the effects of GABA receptor agonists on motor function.
Methods
Animals
All procedures were approved by the University of Arizona Animal Care and Use Committee (Tucson, Arizona) and conformed to the policies, recommendations, and guidelines of the National Institutes of Health and the International Association for the Study of Pain. 18 Male Sprague-Dawley rats (Harlan, Indianapolis, IN) weighing 250–350 g at the time of testing were maintained in a climate-controlled room on a 12-h light–dark cycle and were allowed food and water ad libitum  .
Implantation of Intrathecal Catheter
For intrathecal drug administration, rats were chronically implanted with catheters as described by Yaksh and Rudy. 19 Rats were anesthetized with halothane and placed in stereotactic head holders. The occipital muscles were separated from their insertion on the skull and retracted caudally to expose the cisternal membrane. Polyethylene tubing was passed caudally from the cisterna magna to the level of the lumbar enlargement. Animals were allowed to recover and were examined for evidence of neurologic injury. Animals with evidence of neuromuscular deficits were promptly euthanized.
Spinal Nerve Ligation Injury
The L5 and L6 spinal nerves were ligated as described by Kim and Chung. 12 During halothane anesthesia, the dorsal vertebral column was surgically exposed from L4 to S2. The L5 and L6 spinal nerves were exposed and tightly ligated distal to the dorsal root ganglion using 4-0 silk suture. The incisions were closed, and the animals were allowed to recover for at least 4 days. Testing of drug effects was performed 4–14 days after nerve injury. Rats that showed motor dysfunction or in which tactile allodynia did not develop (less than 2% of animals) were excluded from further testing.
Drug Administration
Isoguvacine hydrochloride, (±)-baclofen, (+)-bicuculline, and phaclofen were obtained from RBI/Sigma (Natick, MA). SCH50911 was obtained from Tocris Cookson Inc. (Ellisville, MO). All drugs were administered through the intrathecal catheter. Testing was performed 30 min after administration of drug or vehicle.
Nociceptive Testing
The tail was placed in a heated water bath maintained at 52°C. Latency to withdrawal was determined before drug administration and 30 min after drug injection. A 12-s cutoff was used to prevent tissue damage.
Testing of Allodynia and Hyperalgesia
The assessment of tactile allodynia (decreased threshold to paw withdrawal after probing with normally innocuous mechanical stimuli) consisted of testing the withdrawal threshold of the paw (ipsilateral to the side of surgery in nerve-injured animals) in response to probing with a series of calibrated von Frey filaments. Each filament was applied perpendicularly to the plantar surface of the paw of rats held in suspended wire-mesh cages. Withdrawal threshold was determined by sequentially increasing and decreasing the stimulus strength (the “up-and-down” method), and the data were analyzed using the nonparametric method of Dixon, as described by Chaplan et al.  13 
Thermal hyperalgesia of the hind paws was tested as described by Hargreaves et al.  20 Rats were allowed to acclimate within acrylic enclosures on a clear glass plate maintained at 30°C. A radiant heat source (high-intensity projector lamp) was focused onto the plantar surface of the hind paw. Activation of the heat source activated a timer that stopped when withdrawal of the paw was detected with a photodetector. A maximal cutoff of 40 s was used to prevent tissue damage.
Testing of Motor Function
Animals were trained to ambulate on a rotarod device (Columbus Instruments International, Columbus, OH) until all could remain on the device for a duration of 180 s at a speed of 10 revolutions per minute. They were again tested after drug administration, and the time they were able to remain on the device without falling was recorded. A maximal cutoff time of 180 s was used.
Data Analysis
Reversal of allodynia was expressed as the percent of the maximum possible effect (MPE) using the formula:
where WT is the withdrawal threshold obtained experimentally, CT (control threshold) is the baseline value before drug administration, and CO is the cutoff value (15 g). Antinociception was also expressed as a percent of the MPE, calculated by the formula described. Reversal of thermal hyperalgesia was defined as return of paw withdrawal latency toward the values observed in nonoperated animals. Antinociception to thermal stimuli was defined as further increases in paw withdrawal latency beyond the baseline values of nonoperated animals.
Results
Intrathecal administration of the GABAAreceptor– selective antagonist bicuculline decreased the threshold for withdrawal from tactile stimuli by 89%, resulting in tactile allodynia (fig. 1). The GABABreceptor–selective antagonist phaclofen decreased the threshold for withdrawal from tactile stimuli by 92%. Similarly, bicuculline reduced the latency to withdrawal from a radiant thermal stimulus by 38%, resulting in thermal hyperalgesia. Phaclofen reduced thermal withdrawal latency by 36%.
Fig. 1. Tactile allodynia produced by intrathecal bicuculline (A  ) or phaclofen (B  ). Thermal hyperalgesia produced by intrathecal bicuculline (C  ) or phaclofen (D  ). Data are expressed as mean ± standard error of the mean. N = 6 per group.
Fig. 1. Tactile allodynia produced by intrathecal bicuculline (A 
	) or phaclofen (B 
	). Thermal hyperalgesia produced by intrathecal bicuculline (C 
	) or phaclofen (D 
	). Data are expressed as mean ± standard error of the mean. N = 6 per group.
Fig. 1. Tactile allodynia produced by intrathecal bicuculline (A  ) or phaclofen (B  ). Thermal hyperalgesia produced by intrathecal bicuculline (C  ) or phaclofen (D  ). Data are expressed as mean ± standard error of the mean. N = 6 per group.
×
Baseline tail-flick latency was 4.1 ± 0.7 s (mean ± SD). Intrathecal administration of the GABAAreceptor–selective agonist isoguvacine did not affect antinociception, as assessed by tail-flick latency (fig. 2). In contrast, intrathecal administration of the GABABreceptor–selective agonist baclofen prolonged tail-flick latency to 100% of the MPE.
Fig. 2. Thermal antinociception produced by intrathecal isoguvacine (A  ) or baclofen (B  ). Data are expressed as mean ± standard error of the mean. N = 6 per group. MPE = maximum possible effect.
Fig. 2. Thermal antinociception produced by intrathecal isoguvacine (A 
	) or baclofen (B 
	). Data are expressed as mean ± standard error of the mean. N = 6 per group. MPE = maximum possible effect.
Fig. 2. Thermal antinociception produced by intrathecal isoguvacine (A  ) or baclofen (B  ). Data are expressed as mean ± standard error of the mean. N = 6 per group. MPE = maximum possible effect.
×
Ligation of the L5 and L6 spinal nerves resulted in tactile allodynia, manifested by a decrease in withdrawal threshold to tactile stimuli from 11 ± 4 to 1.9 ± 2.1 g (mean ± SD;P  < 0.05, Student t  test). Intrathecal administration of isoguvacine reversed this tactile allodynia to a maximum of 75% MPE (fig. 3). The effect of isoguvacine was completely (> 80%) reversed by intrathecal bicuculline or phaclofen. Intrathecal administration of baclofen reversed tactile allodynia to a maximum of 90% MPE (fig. 4). This effect was reversed by intrathecal phaclofen (55% effect), SCH 50911 (85% effect), and bicuculline (70% effect).
Fig. 3. Reversal of SNL-induced tactile allodynia by intrathecal isoguvacine (A  ). Reversal by bicuculline (B  ) or phaclofen (C  ) of the antiallodynia produced by 10 μg isoguvacine. Data are expressed as mean ± standard error of the mean. N = 6 per group. MPE = maximum possible effect.
Fig. 3. Reversal of SNL-induced tactile allodynia by intrathecal isoguvacine (A 
	). Reversal by bicuculline (B 
	) or phaclofen (C 
	) of the antiallodynia produced by 10 μg isoguvacine. Data are expressed as mean ± standard error of the mean. N = 6 per group. MPE = maximum possible effect.
Fig. 3. Reversal of SNL-induced tactile allodynia by intrathecal isoguvacine (A  ). Reversal by bicuculline (B  ) or phaclofen (C  ) of the antiallodynia produced by 10 μg isoguvacine. Data are expressed as mean ± standard error of the mean. N = 6 per group. MPE = maximum possible effect.
×
Fig. 4. Reversal of spinal nerve ligation–induced tactile allodynia by intrathecal baclofen (A  ). Reversal by phaclofen (B  ), SCH 50911, (C  ) or bicuculline (D  ) of the antiallodynia produced by 1 μg baclofen. Data are expressed as mean ± standard error of the mean. N = 6 per group. MPE = maximum possible effect.
Fig. 4. Reversal of spinal nerve ligation–induced tactile allodynia by intrathecal baclofen (A 
	). Reversal by phaclofen (B 
	), SCH 50911, (C 
	) or bicuculline (D 
	) of the antiallodynia produced by 1 μg baclofen. Data are expressed as mean ± standard error of the mean. N = 6 per group. MPE = maximum possible effect.
Fig. 4. Reversal of spinal nerve ligation–induced tactile allodynia by intrathecal baclofen (A  ). Reversal by phaclofen (B  ), SCH 50911, (C  ) or bicuculline (D  ) of the antiallodynia produced by 1 μg baclofen. Data are expressed as mean ± standard error of the mean. N = 6 per group. MPE = maximum possible effect.
×
Spinal nerve ligation of L5–L6 reduced withdrawal latency to radiant heat from 22 ± 4 to 12 ± 4 s (mean ± SD), indicating thermal hyperalgesia (P  < 0.05, Student t  test). Intrathecal isoguvacine returned withdrawal latency to preoperative values (fig. 5). In contrast, intrathecal baclofen lengthened withdrawal latency beyond preoperative values, reaching the cutoff value of 40 s.
Fig. 5. Reversal of spinal nerve ligation–induced thermal hyperalgesia by intrathecal isoguvacine (A  ) or baclofen (B  ). Data are expressed as mean ± standard error of the mean. N = 6 per group.
Fig. 5. Reversal of spinal nerve ligation–induced thermal hyperalgesia by intrathecal isoguvacine (A 
	) or baclofen (B 
	). Data are expressed as mean ± standard error of the mean. N = 6 per group.
Fig. 5. Reversal of spinal nerve ligation–induced thermal hyperalgesia by intrathecal isoguvacine (A  ) or baclofen (B  ). Data are expressed as mean ± standard error of the mean. N = 6 per group.
×
Intrathecal isoguvacine minimally decreased performance on the rotarod apparatus at antiallodynic and antihyperalgesic doses (fig. 6). In contrast, baclofen decreased duration on the rotarod apparatus by 86%.
Fig. 6. Effects of intrathecal isoguvacine (A  ) or baclofen (B  ) on ambulation. Data are expressed as mean ± standard error of the mean. N = 6 per group.
Fig. 6. Effects of intrathecal isoguvacine (A 
	) or baclofen (B 
	) on ambulation. Data are expressed as mean ± standard error of the mean. N = 6 per group.
Fig. 6. Effects of intrathecal isoguvacine (A  ) or baclofen (B  ) on ambulation. Data are expressed as mean ± standard error of the mean. N = 6 per group.
×
Discussion
GABAAand GABABreceptor antagonists produced tactile allodynia and thermal hyperalgesia, confirming the hypothesis that loss of endogenous GABA tone leads to allodynia and hyperalgesia. Constitutive, endogenous spinal GABA activity may lead to tonic inhibition that prevents innocuous mechanical or thermal stimuli from being perceived as aversive or painful. Tonic inhibition is consistent with the high concentrations of GABA and GABA receptor–containing cells found in dorsal horn. 21,22 It is also consistent with electrophysiologic studies in which GABAAreceptor inhibition resulted in exaggerated spinal responses to afferent input 4–6 and with the observation that GABABreceptor inhibition increased substance P release, as assessed by receptor internalization. 23 
Our finding that the GABAAreceptor–selective antagonist bicuculline produces tactile allodynia when administered spinally to the rat is consistent with previous studies. 3,5,24,25 The production of thermal hyperalgesia by bicuculline supports the hypothesis of Yamamoto and Yaksh, 1 who showed that bicuculline enhanced the thermal hyperalgesia produced by chronic constriction injury and hypothesized that the lesion and its exaggeration by a GABAAantagonist suggest mechanistic homology along a spectrum of progressive severity. The production of tactile allodynia and thermal hyperalgesia by bicuculline is consistent with the finding that mice lacking the β3 subunit of the GABAAreceptor exhibit tactile allodynia and thermal hyperalgesia. 26 Bicuculline-mediated thermal hyperalgesia is not predicted by the results of an electrophysiologic study in which GABAAreceptor inhibition resulted in an exaggerated response to low but not high threshold afferent input, 4 but it is consistent with the results of other electrophysiologic studies in which GABAAreceptor inhibition resulted in increased responsiveness to both low and high threshold afferent input. 6,27 
These observations with spinal GABA antagonists demonstrate that loss of GABA activity at spinal GABAAand GABABreceptors is sufficient to lower mechanical and thermal sensory thresholds. There is evidence that endogenous GABA concentrations may be reduced after peripheral nerve injury. Loss of GABA-like immunoreactivity has been reported after chronic constriction injury of the sciatic nerve. 2 Primary afferent depolarization, a GABA receptor–mediated effect, is decreased after chronic constriction injury. 28 There is also evidence that GABA receptor number or function may be altered after peripheral nerve injury. The sensitivity of dorsal roots to GABA was decreased after sciatic axotomy or crush injury. 29 In contrast, bicuculline increased C-fiber–mediated activity in dorsal horn neurons in spinal nerve–ligated rats but not in control animals, suggesting an increased GABAergic inhibitory tone in spinal cord after nerve injury. 7 Direct examinations of GABA receptor number have also yielded conflicting results. GABABreceptor binding was decreased after sciatic neurectomy, whereas GABAAreceptor binding was increased. 30 In contrast, messenger RNA for the GABAAreceptor was decreased in dorsal root ganglion after L5 SNL. 31 
If nerve injury–induced tactile allodynia and thermal hyperalgesia are produced by loss of endogenous GABA tone, GABAAagonist, GABABagonist, or both should reverse these signs of increased nociceptive sensitivity. In our studies, both isoguvacine and baclofen relieved allodynia and hyperalgesia, consistent with this prediction. Relief of allodynia by spinal GABAAand GABABagonists is consistent with previous studies. Baclofen and muscimol reversed touch-evoked allodynia produced by L5–L6 SNL and mechanical hyperalgesia produced by chronic constriction injury. 14,15 In contrast to findings in the SNL and chronic constriction injury models, intrathecal baclofen did not alleviate pain behaviors in a model of sciatic nerve injury produced by photochemically induced ischemia. 16 
Thermal hyperalgesia produced by peripheral nerve injury seems to have a different spinal pharmacology and neural circuitry than does tactile allodynia. For example, thermal hyperalgesia is much more responsive to intrathecal morphine than is tactile allodynia. 9 The afferent signal encoding tactile allodynia seems to be encoded by large, myelinated primary afferent fibers, whereas the signal encoding thermal hyperalgesia is carried by small, high-threshold unmyelinated fibers. 10 Thermal hyperalgesia seems to be mediated by pathways intrinsic to the spinal cord, whereas tactile allodynia requires the contribution of supraspinal sites. 11 Therefore, examination of both modalities is important when examining the spinal pharmacology of the nerve-injured state.
The ability of GABA receptor agonists to diminish behavioral responses to high-threshold stimuli has been demonstrated, 15,32–38 implying that pathways encoding high-threshold nociceptive input contain GABA receptor. The presence of GABA receptors on pathways encoding high-threshold input provides a mechanism for endogenous spinal GABA to act tonically to down-regulate thermal responsiveness. Disinhibition of these pathways by GABA antagonists would result in thermal hyperalgesia.
Isoguvacine exerted a specific antihyperalgesic effect at the doses used. That is, it relieved thermal hyperalgesia but did not affect thermal nociception. Previous studies using higher doses have shown antinociceptive effects of isoguvacine after intrathecal or epidural administration. 39,40 The reversal of hyperalgesia but not nociception at the doses used in this study may be because a smaller number of GABAAreceptors need to be activated to reverse hyperalgesia than to produce antinociception.
The effects of GABA receptor agonists on nerve injury–induced tactile allodynia were blocked by the appropriate GABA receptor antagonists. The effects of baclofen on tactile allodynia were incompletely prevented by the GABABreceptor–selective antagonist phaclofen. However, the GABABreceptor–selective antagonist SCH50911 fully reversed the effect of baclofen, suggesting that the partial reversal by phaclofen may have been due to incomplete antagonism of baclofen at the GABABreceptor. Bicuculline and phaclofen have been reported to be selective for the GABAAand GABABreceptors, respectively. However, after nerve injury, we were not able to demonstrate significant receptor selectivity of these compounds. Bicuculline was only 10-fold more potent in reversing the actions of isoguvacine than in reversing the actions of baclofen. Similar doses of phaclofen were required to reverse the actions of isoguvacine and baclofen. Because these antagonists decrease withdrawal threshold or latency when administered alone, the apparent blockade of agonist actions may have been due to direct allodynic or hyperalgesic actions of bicuculline or phaclofen, rather than to blockade of the antiallodynic or antihyperalgesic of isoguvacine and baclofen at GABA receptors. With respect to agonist selectivity, our results show that isoguvacine acted only at the GABAAreceptor because it did not produce the antinociceptive or motor effects observed with baclofen.
Analysis of antinociception, antiallodynia, and antihyperalgesia produced by baclofen is complicated by motor effects of the drug. Baclofen impaired performance on the rotarod apparatus in the antinociceptive, antiallodynic, and antihyperalgesic dose ranges, suggesting that motor dysfunction may have impaired withdrawal from the evocative stimuli, thereby mimicking antinociception, antiallodynia, and antihyperalgesia. Impairment of performance on the rotarod apparatus could be due to sensory dysfunction, sedation, loss of motor coordination, or motor weakness. The motor nature of these effects is supported by the qualitative observation that intrathecal baclofen–treated animals seemed to have motor weakness of the hind limbs
Our finding that isoguvacine did not produce significant motor dysfunction is in contrast to previous studies showing that the GABAAagonist muscimol produced motor weakness at analgesic doses. 34,36 This difference suggests that the motor effects of muscimol may have been caused by a mechanism not mediated by the GABAAreceptor. Our results are consistent with those of Hwang and Yaksh 14 who, using a four-point scale of motor deficit scoring, did not observe motor deficits with antiallodynic doses of muscimol. Our results with baclofen are in contrast to those of Hwang and Yaksh, 14 who found that antiallodynic doses of baclofen did not produce visible motor deficits. This may be explained if rotarod testing is a more sensitive test of motor dysfunction than is the observational scale. The lack of specific antiallodynic and antihyperalgesic effects of baclofen at doses less than those producing motor dysfunction is consistent with the limited effectiveness of baclofen in many clinical cases of neuropathic pain.
These data support the hypothesis that neuropathic pain may result from a loss of GABA receptor–mediated inhibition of somatosensory pathways. Our results provide the first demonstration that GABAAand GABABreceptor–selective antagonists produce thermal hyperalgesia and that GABAAand GABABreceptor–selective agonists reverse the thermal hyperalgesia produced by peripheral nerve injury. In parallel, they confirm previous work demonstrating that GABAAand GABABreceptor antagonists agonists produce tactile allodynia and that GABAAand GABABreceptor agonists reverse the tactile allodynia produced by peripheral nerve injury. Study of responses to both mechanical and thermal stimuli is crucial because thermal hyperalgesia has different spinal pharmacology and neural circuitry than does allodynia. 9–11 In addition, our studies used a sensitive, reproducible test of motor function to compare the antiallodynic, antihyperalgesic, and motor effects of GABA receptor agonists.
The specific antiallodynic and antihyperalgesic effects of isoguvacine, which lacks effects on acute nociception or motor function at antiallodynic and antihyperalgesic doses, suggest that GABAAagonists may be efficacious in the treatment of neuropathic pain without sensory or motor side effects.
References
Yamamoto T, Yaksh TL: Effects of intrathecal strychnine and bicuculline on nerve compression-induced thermal hyperalgesia and selective antagonism by MK-801. Pain 1993; 54: 79–84Yamamoto, T Yaksh, TL
Ibuki T, Hama AT, Wang X-T, Pappas GD, Sagen J: Loss of GABA immunoreactivity in the spinal dorsal horn of rats with peripheral nerve injury and promotion of recovery by adrenal medullary grafts. Neuroscience 1997; 76: 845–58Ibuki, T Hama, AT Wang, X-T Pappas, GD Sagen, J
Yaksh TL: Behavioral and autonomic correlates of the tactile evoked allodynia produced by spinal glycine inhibition: Effects of modulatory receptor systems and excitatory amino acid antagonists. Pain 1989; 37: 119–23Yaksh, TL
Sivilotti L, Woolf CJ: The contribution of GABAAand glycine receptors to central sensitization: Disinhibition and touch-evoked allodynia in the spinal cord. J Neurophysiol 1994; 72: 169–79Sivilotti, L Woolf, CJ
Reeve AJ, Dickenson AH, Kerr NC: Spinal effects of bicuculline: modulation of an allodynia-like state by an A1-receptor agonist, morphine, and an NMDA-receptor antagonist. J Neurophysiol 1998; 79: 1494–507Reeve, AJ Dickenson, AH Kerr, NC
Sorkin LS, Puig S, Jones DL: Spinal bicuculline produces hypersensitivity of dorsal horn neurons: Effects of excitatory amino acid antagonists. Pain 1998; 77: 181–90Sorkin, LS Puig, S Jones, DL
Kontinen VK, Stanfa LC, Basu A, Dickenson AH: Electrophysiologic evidence for increased endogenous GABAergic but not glycinergic inhibitory tone in the rat spinal nerve ligation model of neuropathy. A nesthesiology 2001; 94: 333–9Kontinen, VK Stanfa, LC Basu, A Dickenson, AH
Hao J-X, Xu X-J, Weisenfeld-Hallin Z: Intrathecal γ-aminobutyric acidB(GABAB) receptor CGP 35348 induces hypersensitivity to mechanical stimuli in the rat. Neurosci Lett 1994; 182: 299–302Hao, J-X Xu, X-J Weisenfeld-Hallin, Z
Lee YW, Chaplan SR, Yaksh TL: Systemic and supraspinal, but not spinal, opiates suppress allodynia in a rat neuropathic pain model. Neurosci Lett 1995; 199: 111–4Lee, YW Chaplan, SR Yaksh, TL
Ossipov MH, Bian D, Malan TP, Lai J, Porreca F: Lack of involvement of capsaicin-sensitive primary afferents in nerve-ligation injury induced tactile allodynia in rats. Pain 1999; 79: 127–33Ossipov, MH Bian, D Malan, TP Lai, J Porreca, F
Bian D, Ossipov MH, Zhong C, Malan TP, Porreca F: Tactile allodynia, but not thermal hyperalgesia, of the hindlimbs is blocked by spinal transection in rats with nerve injury. Neurosci Lett 1998; 241: 79–82Bian, D Ossipov, MH Zhong, C Malan, TP Porreca, F
Kim SH, Chung JM: An experimental model for peripheral neuropathy produced by segmental spinal nerve ligation in the rat. Pain 1992; 50: 355–363Kim, SH Chung, JM
Chaplan SR, Bach FW, Pogrel JW, Chung M, Yaksh TL: Quantitative assessment of tactile allodynia in the rat paw. J Neurosci Meth 1994; 53: 55–63Chaplan, SR Bach, FW Pogrel, JW Chung, M Yaksh, TL
Hwang JH, Yaksh TL: The effect of spinal GABA receptor agonists on tactile allodynia in a surgically induced neuropathic pain model in the rat. Pain 1997; 70: 15–22Hwang, JH Yaksh, TL
Smith GD, Harrison SM, Birch PJ, Elliot PJ, Malcangio M, Bowery NG: Increased sensitivity to the antinociceptive activity of (±)-baclofen in an animal model of chronic neuropathic, but not chronic inflammatory hyperalgesia. Neuropharmacology 1994; 33: 1103–8Smith, GD Harrison, SM Birch, PJ Elliot, PJ Malcangio, M Bowery, NG
Hao J-X, Xu IS, Xu X-J, Wiesenfeld-Hallin Z: Effects of intrathecal morphine, clonidine and baclofen on allodynia after partial sciatic nerve injury in the rat. Acta Anaesthesiol Scand 1999; 43: 1027–34Hao, J-X Xu, IS Xu, X-J Wiesenfeld-Hallin, Z
Kontinen VK, Dickenson AH: Effects of midazolam in the spinal nerve ligation model of neuropathic pain in rats. Pain 2000; 85: 425–31Kontinen, VK Dickenson, AH
Zimmerman M: Ethical guidelines for investigations of experimental pain in conscious animals. Pain 1983; 16: 109–10Zimmerman, M
Yaksh TL, Rudy TA: Chronic catheterization of the spinal sub-arachnoid space. Physiol Biochem Behav 1976; 17: 1031–6Yaksh, TL Rudy, TA
Hargreaves KM, Dubner R, Brown F, Flores C, Joris J: A new and sensitive method for measuring thermal nociception in cutaneous hyperalgesia. Pain 1988; 32: 77–88Hargreaves, KM Dubner, R Brown, F Flores, C Joris, J
Bowery NG, Hudson AL, Price GW: GABAAand GABABreceptor site distribution in the rat central nervous system. Neuroscience 1987; 20: 365–383Bowery, NG Hudson, AL Price, GW
Todd AG, Sullivan AC: Light microscopic study of the coexistence of GABA-like and glycine-like immunoreactivities in the spinal cord of the rat. J Comp Neurol 1990; 296: 496–505Todd, AG Sullivan, AC
Marvizón JCG, Grady EF, Stefani E, Burnett NW, Mayer EA: Substance P release in the dorsal horn assessed by receptor internalization: NMDA receptors counteract a tonic inhibition by GABABreceptors. Eur J Neurosci 1999; 11: 417–26Marvizón, JCG Grady, EF Stefani, E Burnett, NW Mayer, EA
Ishikawa T, Marsala M, Sakabe T, Yaksh T: Characterization of spinal amino acid release and touch-evoked allodynia produced by spinal glycine or GABAAreceptor antagonist. Neuroscience 2000; 95: 781–6Ishikawa, T Marsala, M Sakabe, T Yaksh, T
McGowan MK, Hammond DL: Antinociception produced by microinjection of L-glutamate into the ventromedial medulla of the rat: Mediation by spinal GABAAreceptors. Brain Res 1993; 620: 86–96McGowan, MK Hammond, DL
Ugarte SD, Homanics GE, Firestone LL, Hammond DL: Sensory thresholds and the antinociceptive effects of GABA receptor agonists in mice lacking the β3 subunit of the GABAAreceptor. Neuroscience 2000; 95: 795–806Ugarte, SD Homanics, GE Firestone, LL Hammond, DL
Peng YB, Lin Q, Willis WD: Effects of GABA and glycine receptor antagonists on the activity and PAG-induced inhibition of rat dorsal horn neurons. Brain Res 1996; 736: 189–201Peng, YB Lin, Q Willis, WD
Laird JM, Bennet GJ: Dorsal root potentials and afferent input to the spinal cord in rats with an experimental peripheral neuropathy. Brain Res 1992; 584: 181–90Laird, JM Bennet, GJ
Kingery WS, Fields RD, Kocsis JD: Diminished dorsal root sensitivity following chronic peripheral nerve injury. Exp Neurol 1988; 100: 478–90Kingery, WS Fields, RD Kocsis, JD
Castro-Lopes JM, Malcangio M, Pan BH, Bowery NG: Complex changes in GABAAand GABABreceptor binding in the spinal cord dorsal horn following peripheral inflammation or neurectomy. Brain Res 1995; 289–297:Castro-Lopes, JM Malcangio, M Pan, BH Bowery, NG
Fukuoka T, Tokunaga A, Kondo E, Miki K, Tachibana T, Noguchi K: Change in mRNAs for neuropeptides and the GABAAreceptor in dorsal root ganglion neurons in a rat experimental neuropathic pain model. Pain 1998; 78: 13–26Fukuoka, T Tokunaga, A Kondo, E Miki, K Tachibana, T Noguchi, K
Wilson PR, Yaksh TL: Baclofen is antinociceptive in the spinal intrathecal space of animals. Eur J Pharmacol 1978; 51: 323–30Wilson, PR Yaksh, TL
Yaksh TL, Reddy SVR: Studies in the primate on the analgesic effects associated with intrathecal actions of opiates, alpha-adrenergic agonists and baclofen. A nesthesiology 1981; 54: 451–67Yaksh, TL Reddy, SVR
Hammond DL, Drower EJ: Effects of intrathecally administered THIP, baclofen and muscimol on nociceptive threshold. Eur J Pharmacol 1984; 103: 121–5Hammond, DL Drower, EJ
Hwang AS, Wilcox GL: Baclofen, γ-aminobutyric acid-B receptors and substance P in the mouse spinal cord. J Pharmacol Exp Ther 1989; 248: 1026–33Hwang, AS Wilcox, GL
Aanonsen LM, Wilcox GL: Muscimol, γ-aminobutyric acid receptors and excitatory amino acids in the mouse spinal cord. J Pharmacol Exp Ther 1989; 248: 1034–8Aanonsen, LM Wilcox, GL
Aran S, Hammond DL: Antagonism of baclofen-induced antinociception by intrathecal administration of phaclofen or 2-hydroxysaclofen, but not δ-aminovaleric acid in the rat. J Pharmacol Exp Ther 1991; 257: 360–8Aran, S Hammond, DL
Nadeson R, Guo Z, Porter V, Gent JP, Goodchild CS:γ-Aminobutyric acidAreceptors and spinally mediated nociception in rats. J Pharmacol Exp Ther 1996; 278: 620–6Nadeson, R Guo, Z Porter, V Gent, JP Goodchild, CS
Kaneko M, Hammond DL: Role of spinal γ-aminobutyric acid-A receptors in formalin-induced nociception in the rat. J Pharmacol Exp Ther 1997; 282: 928–38Kaneko, M Hammond, DL
Watkins LR, McGorry M, Schwartz B, Sisk D, Wiertelak EP, Maier SF: Reversal of spinal cord non-opiate analgesia by conditioned anti-analgesia in the rat. Pain 1997; 71: 237–47Watkins, LR McGorry, M Schwartz, B Sisk, D Wiertelak, EP Maier, SF
Fig. 1. Tactile allodynia produced by intrathecal bicuculline (A  ) or phaclofen (B  ). Thermal hyperalgesia produced by intrathecal bicuculline (C  ) or phaclofen (D  ). Data are expressed as mean ± standard error of the mean. N = 6 per group.
Fig. 1. Tactile allodynia produced by intrathecal bicuculline (A 
	) or phaclofen (B 
	). Thermal hyperalgesia produced by intrathecal bicuculline (C 
	) or phaclofen (D 
	). Data are expressed as mean ± standard error of the mean. N = 6 per group.
Fig. 1. Tactile allodynia produced by intrathecal bicuculline (A  ) or phaclofen (B  ). Thermal hyperalgesia produced by intrathecal bicuculline (C  ) or phaclofen (D  ). Data are expressed as mean ± standard error of the mean. N = 6 per group.
×
Fig. 2. Thermal antinociception produced by intrathecal isoguvacine (A  ) or baclofen (B  ). Data are expressed as mean ± standard error of the mean. N = 6 per group. MPE = maximum possible effect.
Fig. 2. Thermal antinociception produced by intrathecal isoguvacine (A 
	) or baclofen (B 
	). Data are expressed as mean ± standard error of the mean. N = 6 per group. MPE = maximum possible effect.
Fig. 2. Thermal antinociception produced by intrathecal isoguvacine (A  ) or baclofen (B  ). Data are expressed as mean ± standard error of the mean. N = 6 per group. MPE = maximum possible effect.
×
Fig. 3. Reversal of SNL-induced tactile allodynia by intrathecal isoguvacine (A  ). Reversal by bicuculline (B  ) or phaclofen (C  ) of the antiallodynia produced by 10 μg isoguvacine. Data are expressed as mean ± standard error of the mean. N = 6 per group. MPE = maximum possible effect.
Fig. 3. Reversal of SNL-induced tactile allodynia by intrathecal isoguvacine (A 
	). Reversal by bicuculline (B 
	) or phaclofen (C 
	) of the antiallodynia produced by 10 μg isoguvacine. Data are expressed as mean ± standard error of the mean. N = 6 per group. MPE = maximum possible effect.
Fig. 3. Reversal of SNL-induced tactile allodynia by intrathecal isoguvacine (A  ). Reversal by bicuculline (B  ) or phaclofen (C  ) of the antiallodynia produced by 10 μg isoguvacine. Data are expressed as mean ± standard error of the mean. N = 6 per group. MPE = maximum possible effect.
×
Fig. 4. Reversal of spinal nerve ligation–induced tactile allodynia by intrathecal baclofen (A  ). Reversal by phaclofen (B  ), SCH 50911, (C  ) or bicuculline (D  ) of the antiallodynia produced by 1 μg baclofen. Data are expressed as mean ± standard error of the mean. N = 6 per group. MPE = maximum possible effect.
Fig. 4. Reversal of spinal nerve ligation–induced tactile allodynia by intrathecal baclofen (A 
	). Reversal by phaclofen (B 
	), SCH 50911, (C 
	) or bicuculline (D 
	) of the antiallodynia produced by 1 μg baclofen. Data are expressed as mean ± standard error of the mean. N = 6 per group. MPE = maximum possible effect.
Fig. 4. Reversal of spinal nerve ligation–induced tactile allodynia by intrathecal baclofen (A  ). Reversal by phaclofen (B  ), SCH 50911, (C  ) or bicuculline (D  ) of the antiallodynia produced by 1 μg baclofen. Data are expressed as mean ± standard error of the mean. N = 6 per group. MPE = maximum possible effect.
×
Fig. 5. Reversal of spinal nerve ligation–induced thermal hyperalgesia by intrathecal isoguvacine (A  ) or baclofen (B  ). Data are expressed as mean ± standard error of the mean. N = 6 per group.
Fig. 5. Reversal of spinal nerve ligation–induced thermal hyperalgesia by intrathecal isoguvacine (A 
	) or baclofen (B 
	). Data are expressed as mean ± standard error of the mean. N = 6 per group.
Fig. 5. Reversal of spinal nerve ligation–induced thermal hyperalgesia by intrathecal isoguvacine (A  ) or baclofen (B  ). Data are expressed as mean ± standard error of the mean. N = 6 per group.
×
Fig. 6. Effects of intrathecal isoguvacine (A  ) or baclofen (B  ) on ambulation. Data are expressed as mean ± standard error of the mean. N = 6 per group.
Fig. 6. Effects of intrathecal isoguvacine (A 
	) or baclofen (B 
	) on ambulation. Data are expressed as mean ± standard error of the mean. N = 6 per group.
Fig. 6. Effects of intrathecal isoguvacine (A  ) or baclofen (B  ) on ambulation. Data are expressed as mean ± standard error of the mean. N = 6 per group.
×