Carrageenan-induced inflammation of the hind foot provokes a rise of GABA-immunoreactive cells in the rat spinal cord that is prevented by peripheral neurectomy or neonatal capsaicin treatment

Spinal GABA transporter 1 contributes to evoked-pain related behavior but not resting pain after incision injury

The inhibitory function of GABA at the spinal level and its central modulation in the brain are essential for pain perception. However, in post-surgical pain, the exact mechanism and modes of action of GABAergic transmission have been poorly studied. This work aimed to investigate GABA synthesis and uptake in the incisional pain model in a time-dependent manner. Here, we combined assays for mechanical and heat stimuli-induced withdrawal reflexes with video-based assessments and assays for non-evoked (NEP, guarding of affected hind paw) and movement-evoked (MEP, gait pattern) pain-related behaviors in a plantar incision model in male rats to phenotype the effects of the inhibition of the GABA transporter (GAT-1), using a specific antagonist (NO711). Further, we determined the expression profile of spinal dorsal horn GAT-1 and glutamate decarboxylase 65/67 (GAD65/67) by protein expression analyses at four time points post-incision. Four hours after incision, we detected an evoked pain phenotype (mechanical, heat and movement), which transiently ameliorated dose-dependently following spinal inhibition of GAT-1. However, the NEP-phenotype was not affected. Four hours after incision, GAT-1 expression was significantly increased, whereas GAD67 expression was significantly reduced. Our data suggest that GAT-1 plays a role in balancing spinal GABAergic signaling in the spinal dorsal horn shortly after incision, resulting in the evoked pain phenotype. Increased GAT-1 expression leads to increased GABA uptake from the synaptic cleft and reduces tonic GABAergic inhibition at the post-synapse. Inhibition of GAT-1 transiently reversed this imbalance and ameliorated the evoked pain phenotype.

Automated home-cage for the evaluation of innate non-reflexive pain behaviors in a mouse model of inflammatory pain

The failure to develop analgesic drugs is attributed not only to the complex and diverse pathophysiology of pain in humans but also to the poor experimental design and poor preclinical assessment of pain. Although considerable efforts have been devoted to overcoming the relevant problems, many features of the behavioral pain assessment remain to be characterized. For example, a decreased locomotor activity as a common presentation of pain-like behavior has yet to be described. Studies on mice experimentally induced with carrageenan have provided opportunities to explore pain-related behaviors in automated home-cage monitoring. Through this approach, the locomotor activities of mice with carrageenan-induced inflammatory pain can be precisely and objectively captured. Here, we found that the mobile behaviors of mice reduced, and their immobility increased, indicating that carrageenan induction in mice caused a significant decrease in locomotor activity. These non-reflexive pain behaviors were strongly correlated with the reflexive pain behaviors measured via von Frey and plantar tests. Furthermore, the pharmacological intervention using indomethacin improved the locomotor activity of mice with carrageenan-induced pain. Thus, the analysis of the locomotor activity in automated home-cage monitoring is useful for studying the behavioral analgesia and the pharmacological screening of analgesic drugs. The combined evaluation of reflexive and non-reflexive pain behaviors enhances the translational utility of preclinical pain research in rodents.

Effect of Zhongyi paste on inflammatory pain in mice by regulation of the extracellular regulated protein kinases 1/2-cyclooxygenase-2-prostaglandin E2 pathway

Background

Zhongyi paste is a traditional Chinese medicine herbal paste that is externally applied to reduce inflammation and relieve pain.

Methods

An acute foot swelling inflammation model in C57BL/6J mice was established by carrageenan-induced pathogenesis. Zhongyi paste raised the pain threshold and also reduced the degree of swelling in mice with carrageenan-induced foot swelling.

Results

Analysis indicated that serum tumor necrosis factor-alpha, interleukin-1 beta, and prostaglandin E₂ (PGE₂) cytokine levels and PGE₂ levels in the paw tissue of the mice were decreased by Zhongyi paste treatment. The quantitative polymerase chain reaction and western blot results showed that Zhongyi paste downregulated the mRNA and protein expression of extracellular signal-regulated kinase 1/2 (ERK1/2), and cyclooxygenase-2 (COX-2), and also downregulated the mRNA expression of PGE₂. At the same time, the Zhongyi paste exerted a stronger effect as an external drug than that of indomethacin, which is an oral drug, and voltaren, which is an externally applied drug.

Conclusions

Our results indicated that Zhongyi paste is a very effective drug to reduce inflammatory swelling of the foot, and its mechanism of action is related to regulation of the ERK1/2–COX-2–PGE₂ pathway.

Synaptic Homeostasis and Allostasis in the Dentate Gyrus Caused by Inflammatory and Neuropathic Pain Conditions

It has been generally accepted that pain can cause imbalance between excitation and inhibition (homeostasis) at the synaptic level. However, it remains poorly understood how this imbalance (allostasis) develops in the CNS under different pain conditions. Here, we analyzed the changes in both excitatory and inhibitory synaptic transmission and modulation of the dentate gyrus (DG) under two pain conditions with different etiology and duration. First, it was revealed that the functions of the input-output (I/O) curves for evoked excitatory postsynaptic currents (eEPSCs) following the perforant path (PP) stimulation were gained under both acute inflammatory and chronic neuropathic pain conditions relative to the controls. However, the functions of I/O curves for the PP-evoked inhibitory postsynaptic currents (eIPSCs) differed between the two conditions, namely it was greatly gained under inflammatory condition, but was reduced under neuropathic condition in reverse. Second, both the frequency and amplitude of miniature IPSCs (mIPSCs) were increased under inflammatory condition, however a decrease in frequency of mIPSCs was observed under neuropathic condition. Finally, the spike discharge of the DG granule cells in response to current injection was significantly increased by neuropathic pain condition, however, no different change was found between inflammatory pain condition and the control. These results provide another line of evidence showing homeostatic and allostatic modulation of excitatory synaptic transmission by inhibitory controls under different pathological pain conditions, hence implicating use of different therapeutic approaches to maintain the homeostasis between excitation and inhibition while treating different conditions of pathological pain.

Participation of central GABAA receptors in the trigeminal processing of mechanical allodynia in rats

Here we investigated the central processing mechanisms of mechanical allodynia and found a direct excitatory link with low-threshold input to nociceptive neurons. Experiments were performed on male Sprague-Dawley rats weighing 230-280 g. Subcutaneous injection of interleukin 1 beta (IL-1β) (1 ng/10 µL) was used to produce mechanical allodynia and thermal hyperalgesia. Intracisternal administration of bicuculline, a gamma aminobutyric acid A (GABA_A) receptor antagonist, produced mechanical allodynia in the orofacial area under normal conditions. However, intracisternal administration of bicuculline (50 ng) produced a paradoxical anti-allodynic effect under inflammatory pain conditions. Pretreatment with resiniferatoxin (RTX), which depletes capsaicin receptor protein in primary afferent fibers, did not alter the paradoxical anti-allodynic effects produced by the intracisternal injection of bicuculline. Intracisternal injection of bumetanide, an Na-K-Cl cotransporter (NKCC 1) inhibitor, reversed the IL-1β-induced mechanical allodynia. In the control group, application of GABA (100 µM) or muscimol (3 µM) led to membrane hyperpolarization in gramicidin perforated current clamp mode. However, in some neurons, application of GABA or muscimol led to membrane depolarization in the IL-1β-treated rats. These results suggest that some large myelinated Aβ fibers gain access to the nociceptive system and elicit pain sensation via GABA_A receptors under inflammatory pain conditions.

Modulation of peripheral inflammation by the spinal cord

The central nervous system, and the spinal cord in particular, is involved in multiple mechanisms that influence peripheral inflammation. Both pro- and anti-inflammatory feedback loops can involve just the peripheral nerves and spinal cord or can also include more complex, supraspinal structures such as the vagal nuclei and the hypothalamic-pituitary axis. Analysis is complicated by the fact that inflammation encompasses a constellation of end points from simple edema to changes in immune cell infiltration and pathology. Whether or not any of these individual elements is altered by any potential mechanism is determined by a complex algorithm including, but not limited to, chronicity of the inflammation, tissue type, instigating stimulus, and state/tone of the immune system. Accordingly, the pharmacology and anatomical substrate of spinal cord modulation of peripheral inflammation are discussed with regard to peripheral tissue type, inflammatory insult (initiating stimulus), and duration of the inflammation.

Reduced GABAA receptor α6 expression in the trigeminal ganglion enhanced myofascial nociceptive response

Activation of the GABAA receptor results in inhibition of neuronal activity. One subunit of this multi-subunit receptor termed alpha 6 (Gabrα6) contributed to inflammatory temporomandibular joint (TMJ) nociception but TMJ disorders often include myofascial pain. To address Gabrα6 role in myofascial pain we hypothesized that Gabrα6 has an inhibitory role in myofascial nociceptive responses similar to inflammatory TMJ arthritis. To test this hypothesis a, myofascial nociceptive response was induced by placing a ligature bilaterally on the tendon attachment of the anterior superficial part of a male rat's masseter muscle. Four days after ligature placement Gabrα6 expression was reduced by infusing the trigeminal ganglia (TG) with small interfering RNA (siRNA) having homology to either the Gabrα6 gene (Gabrα6 siRNA) or no known gene (control siRNA). After siRNA infusion nociceptive behavioral responses were measured, i.e., feeding behavior and head withdrawal after pressing upon the region above the ligature with von Frey filaments. Neuronal activity in the TG and trigeminal nucleus caudalis and upper cervical region (Vc-C1) was measured by quantitating the amount of phosphorylated extracellular signal-regulated kinase (p-ERK). Total Gabrα6 and GABAA receptor contents in the TG and Vc-C1 were determined. Gabrα6 siRNA infusion reduced Gabrα6 and GABAA receptor expression and significantly increased the nociceptive response in both nociceptive assays. Gabrα6 siRNA infusion also significantly increased TG p-ERK expression of the ligated rats. From these results we conclude GABAA receptors consisting of the Gabrα6 subunit inhibit TG nociceptive sensory afferents in the trigeminal pathway and have an important role in the regulation of myofascial nociception.

Reduced GABA(A) receptor α6 expression in the trigeminal ganglion alters inflammatory TMJ hypersensitivity

Trigeminal ganglia neurons express the GABA(A) receptor subunit alpha 6 (Gabrα6) but the role of this particular subunit in orofacial hypersensitivity is unknown. In this report the function of Gabrα6 was tested by reducing its expression in the trigeminal ganglia and measuring the effect of this reduction on inflammatory temporomandibular joint (TMJ) hypersensitivity. Gabrα6 expression was reduced by infusing the trigeminal ganglia of male Sprague Dawley rats with small interfering RNA (siRNA) having homology to either the Gabrα6 gene (Gabrα6 siRNA) or no known gene (control siRNA). Sixty hours after siRNA infusion the rats received a bilateral TMJ injection of complete Freund's adjuvant to induce an inflammatory response. Hypersensitivity was then quantitated by measuring meal duration, which lengthens when hypersensitivity increases. Neuronal activity in the trigeminal ganglia was also measured by quantitating the amount of phosphorylated ERK. Rats in a different group that did not have TMJ inflammation had an electrode placed in the spinal cord at the level of C1 sixty hours after siRNA infusion to record extracellular electrical activity of neurons that responded to TMJ stimulation. Our results show that Gabrα6 was expressed in both neurons and satellite glia of the trigeminal ganglia and that Gabrα6 positive neurons within the trigeminal ganglia have afferents in the TMJ. Gabrα6 siRNA infusion reduced Gabrα6 gene expression by 30% and significantly lengthened meal duration in rats with TMJ inflammation. Gabrα6 siRNA infusion also significantly increased p-ERK expression in the trigeminal ganglia of rats with TMJ inflammation and increased electrical activity in the spinal cord of rats without TMJ inflammation. These results suggest that maintaining Gabrα6 expression was necessary to inhibit primary sensory afferents in the trigeminal pathway and reduce inflammatory orofacial nociception.

GABA-receptor control of the amplitude and duration of the neuronal responses to formalin in the rat spinal cord

The GABAergic inhibitory system in the dorsal horn of the spinal cord has been implicated in the modulation of pain, including the control of nociceptive transmission during inflammation. This electrophysiological study examined the effects of the GABAA and GABAB receptor antagonists, bicuculline and CGP35348, on the magnitude and duration of the formalin response. The responses of spinal nociceptive dorsal horn neurones to subcutaneous injection of formalin into the hindpaw in the anaesthetized rat were recorded. Both phases of the formalin response were monitored, and the antagonists were administered either simultaneously with formalin or 50 min after injection of formalin. Bicuculline (50 microg), the GABAA antagonist, administered simultaneously with formalin significantly increased the magnitude of the overall response, especially the second phase, and also abolished the silent interphase period. In addition, 50 min after injection of formalin, bicuculline increased the duration of the second phase in a dose-dependent manner. CGP35348 (250 microg), the GABAB antagonist, administered 50 min after injection of formalin also increased the duration of the second phase significantly, but had no effect on the magnitude of the response or the silent interphase when administered simultaneously with formalin. These results show that GABAA- and GABAB-receptor-mediated inhibitions are involved in controlling the duration of the second phase of the formalin response, and that GABAA-receptor-mediated inhibition also contributes to the manifestation of the silent interphase period and the magnitude of the second phase. Thus, GABA neurones are critical in determining the level and duration of nociceptive information transmitted through the spinal cord during inflammation.

Attenuation of pain-related behavior evoked by injury through blockade of neuropeptide Y Y2 receptor

Neuropeptide Y (NPY) has an important but still insufficiently defined role in pain modulation. We therefore examined the ability of NPY to modulate experimentally induced neuropathic pain by injecting it directly into dorsal root ganglion (DRG) immediately following spinal nerve ligation (SNL) injury. We have found that this application exacerbates pain-related behavior induced by SNL in a modality-specific fashion. When saline was injected after SNL, the expected increase in hyperalgesia responses to needle stimulation was present on the 8th postoperative day. When we injected NPY, hyperalgesic responses were increased in a manner similar to the SNL/saline group. To characterize NPY action, specific Y1 and Y2 antagonists were also delivered directly to DRG, which revealed that behavioral actions of NPY were abolished by Y2 receptor antagonist. We tested whether NPY effects were the result of its role in immunity by immunohistochemical staining for glial fibrillary acidic protein, in order to identify activation of DRG satellite cells and dorsal horn astrocytes. Exacerbation of pain-related behavior following NPY injection was accompanied by astrocyte activation in ipsilateral dorsal horn and with satellite cells activation in the DRG proximal to injury. This activation was reduced following Y2 receptor antagonist application. These findings indicate an important link between pain-related behavior and neuroimmune activation by NPY through its Y2 receptor.

Intrathecally injected morphine inhibits inflammatory paw edema: the involvement of nitric oxide and cyclic-guanosine monophosphate

BACKGROUND

Morphine can inhibit inflammatory edema in experimental animals. The mechanisms and sites by which opioids exert this effect are still under debate. Since the spinal level is a site for modulation of the neurogenic component of inflammation, we investigated the effect of intrathecal (i.t.) administration of morphine, and the involvement of spinal nitric oxide (NO)/cyclic-guanosine monophosphate-GMP pathway in carrageenan (CG)-induced paw edema.

METHODS

Male Wistar rats received i.t. injections of drugs (20 microL) 30 min before paw stimulation with CG (150 microg). Edema was measured as paw volume increase (mL), and neutrophil migration was evaluated indirectly by myeloperoxidase (MPO) assay.

RESULTS

Morphine (37, 75, and 150 nmol) inhibited inflammatory edema, but had no effect on MPO activity. Coinjection with naloxone (64 nmol) reversed the effect of morphine. The corticosteroid synthesis inhibitor, aminoglutethimide (50 mg/kg, v.o.), administered 90 min before morphine injection did not modify its antiedematogenic effect. Low doses of the NO synthase inhibitor, N(omega)-nitro-L-arginine (L-NNA; 10 and 30 pmol) increased, while higher doses (3 and 30 nmol) inhibited edema. The guanylate cyclase inhibitor 1H-oxadiazolo[4,3-a]quinoxalin-1-one (ODQ; 21 and 42 nmol) increased, while the phosphodiesterase type 5 inhibitor sildenafil (0.15 and 1.5 nmol) inhibited paw edema. Coadministration of a subeffective dose of L-NNA (3 pmol) or ODQ (10 nmol) with morphine prevented its antiedematogenic effect, but sildenafil (0.15 nmol) rendered a subeffective dose of morphine effective (18 nmol). ODQ also prevented the antiedematogenic effect of the NO donor S-nitroso-N-acetyl-penicilamine.

CONCLUSION

These results support the idea that morphine can act on opioid receptors at the spinal level to produce antiedematogenic, and that the NO/cGMP pathway seems to be an important mediator in this effect.

Peripheral inflammation undermines the plasticity of the isolated spinal cord

Peripheral capsaicin treatment induces molecular changes that sensitize the responses of nociceptive neurons in the spinal dorsal horn. The current studies demonstrate that capsaicin also undermines the adaptive plasticity of the spinal cord, rendering the system incapable of learning a simple instrumental task. In these studies, male rats are transected at the second thoracic vertebra and are tested 24 to 48 hours later. During testing, subjects receive shock to one hindleg when it is extended (controllable stimulation). Rats quickly learn to maintain the leg in a flexed position. Rats that have been injected with capsaicin (1% or 3%) in the hindpaw fail to learn, even when tested on the leg contralateral to the injection. This learning deficit lasts at least 24 hours. Interestingly, training with controllable electrical stimulation prior to capsaicin administration protects the spinal cord against the maladaptive effects. Rats pretrained with controllable stimulation do not display a learning deficit or tactile allodynia. Moreover, controllable stimulation, combined with naltrexone, reverses the capsaicin-induced deficit. These data suggest that peripheral inflammation, accompanying spinal cord injuries, might have an adverse effect on recovery.

Vanilloid receptor-1 (TRPV1)-dependent activation of inhibitory neurotransmission in spinal substantia gelatinosa neurons of mouse

Inhibitory neurotransmission in spinal cord dorsal horn is mainly mediated by gamma-amino butyric acid (GABA) and glycine. By patch clamp recordings and correlative immunocytochemistry, we studied here the effect of 2 microM capsaicin-induced vanilloid receptor-1 (TRPV1) activation on IPSCs in spinal lamina II neurons from post-natal mice. Specificity was confirmed after pre-incubation with the competitive antagonist SB366791 (10 microM). After a single capsaicin pulse, an intense increase of spontaneous IPSC (sIPSC) frequency was observed in the presence of NBQX 10 microM (62/81 neurons; approximately 76%) or NBQX 10 microM + AP-5 20-100 microM (27/42 neurons; approximately 64%). Only a subpopulation (approximately 40%) of responsive neurons showed a significant amplitude increase. Seventy-two percent of the neurons displayed pure GABA(A) receptor-mediated sIPSCs, whereas the remaining ones showed mixed GABAergic/glycinergic events. After two consecutive capsaicin pulses, frequency rises were very similar, and both significantly higher than controls. When the second pulse was given in the presence of 4 microM L732,138, a selective antagonist of the substance P (SP) preferred receptor NK1, we observed a significant loss in frequency increase (63.90% with NBQX and 52.35% with NBQX + AP-5). TTX (1 microM) largely (approximately 81.5%) blocked the effect of capsaicin. These results show that TRPV1 activation on primary afferent fibers releases SP. The peptide then excites inhibitory neurons in laminae I, III and IV, leading to an increased release of GABA/glycine in lamina II via a parallel alternative pathway to glutamate.

Instrumental learning within the spinal cord: underlying mechanisms and implications for recovery after injury

Using spinally transected rats, research has shown that neurons within the L4-S2 spinal cord are sensitive to response-outcome (instrumental) relations. This learning depends on a form of N-methyl-D-aspartate (NMDA)-mediated plasticity. Instrumental training enables subsequent learning, and this effect has been linked to the expression of brain-derived neurotrophic factor. Rats given uncontrollable stimulation later exhibit impaired instrumental learning, and this deficit lasts up to 48 hr. The induction of the deficit can be blocked by prior training with controllable shock, the concurrent presentation of a tonic stimulus that induces antinociception, or pretreatment with an NMDA or gamma-aminobutyric acid-A antagonist. The expression of the deficit depends on a kappa opioid. Uncontrollable stimulation enhances mechanical reactivity (allodynia), and treatments that induce allodynia (e.g., inflammation) inhibit learning. In intact animals, descending serotonergic neurons exert a protective effect that blocks the adverse consequences of uncontrollable stimulation. Uncontrollable, but not controllable, stimulation impairs the recovery of function after a contusion injury.

Effects of GABAA receptor inhibition on response properties of barrel cortical neurons in C-Fiber-depleted rats

C-fiber depletion results in expansion of low threshold somatosensory mechanoreceptive fields. In this study, we investigated the role of intact C-fibers in GABAA-mediated inhibition in barrel cortical neurons. We used electronically controlled mechanical stimulation of whiskers to quantitatively examine the responses of barrel cells to whisker displacements. After systemic injection of picrotoxin neuronal responses were recorded at 5 min intervals for 20 min and then at 10 min intervals for 100 min. Picrotoxin injection caused a 3-fold increase in response magnitude of adjacent whisker stimulation and 1.4-fold increase in response magnitude of principal whisker stimulation with a maximum enhancement 50 min after the injection. There was no significant change in spontaneous activity following picrotoxin injection. The response enhancement and receptive field expansion observed in normal rats were completely absent in the C-fiber-depleted rats. These results suggest that the GABAA-mediated inhibition that modulates the receptive field functional organization of the barrel cortex depends on intact C-fibers.

Effect of respiratory syncytial virus on apnea in weanling rats

Apnea is a common complication of respiratory syncytial virus (RSV) infection in young infants. The purpose of this study was to determine whether this infection affects apnea triggered by sensorineural stimulation in weanling rats. We also studied which neurotransmitters are involved in this response and whether passive prophylaxis with a specific neutralizing antibody (palivizumab) confers protection against it. Weanling rats were inoculated intranasally with RSV, adenovirus, or virus-free medium. Changes in respiratory rate and apnea in response to nerve stimulation with increasing doses of capsaicin were measured by plethysmography. Capsaicin-induced apnea was significantly longer in RSV-infected rats at postinoculation days 2 (upper airways infection) and 5 (lower airways infection), and apnea-related mortality occurred only in the RSV-infected group. Reduction in the duration of apnea was observed after selective inhibition of central gamma-aminobutyric acid (GABA) type A receptors and neurokinin type 1 receptors for substance P. Prophylactic palivizumab protected against apnea and apnea-related mortality. These results suggest that sensorineural stimulation during RSV infection is associated with the development of apnea and apnea-related death in early life, whose mechanism involves the release of GABA acting on central GABA type A receptors and substance P acting on neurokinin type 1 receptors.

GABA(A) receptor activation is involved in noncontingent shock inhibition of instrumental conditioning in spinal rats

Previous work has demonstrated that the spinal cord, isolated from higher neural structures, can support a simple form of instrumental learning. Furthermore, preexposure to uncontrollable (noncontingent) shock to the leg or tail inhibits this form of learning. The present study explores the role of GABA(A) receptor modulation on this inhibitory effect in spinal cord-transected rats. Intrathecal administration of the GABA(A) receptor antagonist bicuculline blocked induction and expression of the inhibition. The GABA(A) receptor agonist muscimol inhibited learning in a dose-dependent manner. However, this effect was transient and showed no additivity with shock. The findings suggest that GABA(A) receptor activation may work like a pharmacological switch that is activated by noncontingent shock to inhibit instrumental conditioning within the spinal cord.

The effects of sympathectomy on capsaicin-evoked fos expression of spinal dorsal horn GABAergic neurons

Electrophysiological studies have suggested that activity of spinal GABAergic interneurons can be enhanced following intradermal injection of capsaicin (CAP). This activity is proposed to be involved in the generation of dorsal root reflexes (DRRs) that contribute to neurogenic inflammation. We have recently reported that NMDA or non-NMDA antagonists by intrathecal pretreatment attenuate the increased Fos expression in spinal dorsal horn GABAergic neurons after intradermal injection of CAP in rats. Sympathetic efferents have been suggested to modulate inflammatory pain possibly by interactions with primary afferent terminals. In electrophysiological studies by our group, enhancement of the CAP-induced DRRs could be prevented by surgical sympathectomy and blocked by intraarterial pretreatment of the foot with alpha(1)- but not by alpha(2)-adrenoceptor antagonists. In order to determine morphologically if surgical sympathectomy changes the expression of Fos in GABAergic neurons in the lumbosacral spinal cord induced by CAP injection, further experiments were performed using immunofluorescence double-labeling staining at 30 min following CAP or vehicle injection into the glabrous skin of one hind paw of anesthetized rats both in sham-operated and sympathectomized animals. Our results showed that the proportion of Fos-positive GABAergic neuronal profiles was significantly increased following CAP injection (48.8+/-4.76%) compared to vehicle injection (23.8+/-5.1%) in laminae I-V on the ipsilateral side (P<0.05). However, when sympathetic efferents were removed surgically 7-10 days prior to the experiment (n=6), only 32.07+/-9.03% of GABA-immunoreactive neuronal profiles were stained for Fos following CAP injection, a significant reduction in the CAP-evoked Fos-staining of GABAergic neurons after surgical sympathectomy. These findings support our previous electrophysiological studies that GABAergic neurons take part in nociceptive processing within the spinal dorsal horn and suggest that sympathetic efferents may affect nociceptive transduction in the periphery.

Expression of 5-HT1A receptor mRNA in rat lumbar spinal dorsal horn neurons after peripheral inflammation

The present study observed the expression of the 5-hydroxytryptamine (5-HT) (1A) receptor mRNA in the lumbar spinal dorsal horn neurons following carrageenan inflammation using in situ hybridization (ISH). We also studied the co-localization of 5-HT(1A) receptor mRNA and gamma-amino butyric acid (GABA) or enkephalin (ENK) immunoreactivities using a combined fluorescent ISH and immunofluorescent histochemical double-staining technique. The finding of this study demonstrated that 5-HT(1A) receptor mRNA was widely distributed in the spinal dorsal horn with the highest density in laminae III-VI. Following carrageenan-induced inflammation, the 5-HT(1A) receptor mRNA expression in all layers of ipsilateral dorsal horn was significantly enhanced, and the peak occurred after 8h. Furthermore, the number of 5-HT(1A) receptor mRNA and GABA or ENK immunoreactive double-labeled cells was also markedly increased 8h after carrageenan injection. These findings suggested that following peripheral inflammation, the synthesis of 5-HT(1A) receptor was increased in the lumbar spinal dorsal horn neurons, especially in spinal GABA and ENK neurons.

Increased synaptosomal [3H] GABA uptake in the rat brainstem after facial carrageenan injections

The aim of the present study was to quantify synaptosomal [(3)H] gamma aminobutyric acid (GABA) uptake in the rat brainstem after facial carrageenan injections. Synaptosomal preparations from the brainstem of rats that had received one or four facial carrageenan injections showed greater GABA binding on the side of the brainstem ipsilateral to the carrageenan injection than on the contralateral side when compared to saline injected controls. In contrast, no difference in GABA binding between the injected and contralateral sides was observed in the same synaptosomal preparations that had been treated with GABA uptake inhibitors NNC-711, beta-alanine, or nipecotic acid. The difference between GABA binding in the absence of the GABA uptake inhibitor and GABA binding in a portion from the same synaptosomal preparation which had been incubated with the GABA uptake inhibitor was obtained to represent [(3)H] GABA binding to GABA transporters/transporter mediated [(3)H] GABA uptake. A significantly greater GABA uptake was observed on the side of the brainstem ipsilateral to the carrageenan injection(s) than on the contralateral side. A consequence of the observed increase in GABA uptake is that it could reduce the amount of GABA in the synaptic cleft. This could influence the transmission of nociceptive input from primary afferents to secondary neurons in the spinal trigeminal nucleus and could be a contributing factor in the development of hyperalgesia after carrageenan injections or other chronic inflammatory conditions.

Gabapentin potentiates N-methyl-D-aspartate receptor mediated currents in rat GABAergic dorsal horn neurons

We previously reported that gabapentin (GBP), a widely prescribed analgesic, enhances N-methyl-aspartate (NMDA) receptor mediated currents only when the intracellular level of protein kinase C is elevated. However, it is unclear how the potentiation of NMDA responses by GBP can lead to pain relief. To resolve this issue, we combined immunocytochemical and patch recording techniques to study the actions of GBP on NMDA receptors in dorsal horn cells isolated from rats with inflammation and to determine the gamma-aminobutyric acid (GABA) content in the recorded cells. We found that all GBP-responsive cells are GABA-immunoreactive and none of the GABA-negative neurons respond to GBP. Thus, GBP appears to enhance NMDA currents in GABAergic neurons. These observations suggest that GBP exerts its antinociceptive action by increasing the activity of these inhibitory neurons.

Activation of spinal ORL-1 receptors prevents acute cutaneous neurogenic inflammation: role of nociceptin-induced suppression of primary afferent depolarization

Neurogenic inflammation is an inflammatory response of peripheral tissue to vasoactive substances released from sensory afferent terminals. It can be triggered via a local axon reflex and by dorsal root reflex (DRR) activity involving the spinal cord. Nociceptin, an endogenous ligand for the opioid receptor-like (ORL-1) G-protein coupled receptor, has been found to inhibit the local axon reflex-mediated neurogenic inflammation by suppressing the release of vasoactive neuropeptides from sensory afferent terminals. The present study was to explore the role of spinal ORL-1 receptors in the modulation of DRR-induced neurogenic inflammation. We first examined the effect of nociceptin on DRR by recording dorsal root potentials (DRPs) and the associated antidromic discharges, evoked by electrical stimulation of an adjacent dorsal root in an in vitro neonatal rat spinal cord preparation. Nociceptin reversibly inhibited the DRP in a concentration-dependent manner (IC50: approximately 45 nM, maximal inhibition: approximately 50%), an effect that was antagonized by the ORL-1 receptor antagonist, J-113397. Neurochemical studies demonstrated that nociceptin (10 microM) also produced an approximately 40% reduction in gamma amino butyric acid (GABA) release evoked by electrical stimulation of neonatal rat spinal cord slices. On the other hand, nociceptin had no effect on exogenous GABA-evoked DRP. These findings suggest that the nociceptin-induced inhibition of the DRP is most likely due to the suppression of GABA release, the principle transmitter mediating DRP, from GABAergic neurons that are pre-synaptic to primary afferent terminals. Finally, in order to explore the physiological significance of such modulation in a fully integrated system, we evaluated the effect of intrathecally administered nociceptin on capsaicin-induced acute cutaneous neurogenic inflammation in rat hind paw, quantified by examining the degree of paw edema in anesthetized rats. The magnitude of capsaicin-induced increase of paw thickness was reduced by approximately 50% from 31+/-1.34% (n=6) to 15+/-1.63% (n=8; P<0.05) by nociceptin (10 micromol). We conclude that spinal ORL-1 receptors can modulate neurogenic inflammation by suppressing the GABAergic neuronal activity in the dorsal horn that is responsible for generating DRRs.

Antihyperalgesic effects of intrathecal neuropeptide Y during inflammation are mediated by Y1 receptors

Inflammation induces an up-regulation of neuropeptide tyrosine (NPY) and its receptors in the dorsal horn, suggesting an important role in nociceptive transmission. Our initial studies revealed that NPY dose-dependently increased hotplate response latency, and to a lesser degree, thermal paw withdrawal latency (PWL); these effects occurred at doses that affect neither motor coordination (as assessed by the rotarod test) nor paw skin temperature. We next evaluated the behavioral effects of intrathecal administration of NPY and NPY antagonists with the aim of assessing the contribution of NPY to correlates of persistent nociception associated with the unilateral plantar injection of carrageenan or complete Freund's adjuvant (CFA). NPY robustly and dose-dependently increased PWL on the side ipsilateral to carrageenan injection, with only a small effect on the contralateral side. Similarly, NPY (30 microg) produced a large and long-lasting increase in PWL on the side ipsilateral to CFA injection (140% change), with only a small effect on the contralateral side (25% change). The ipsilateral effect of NPY was completely inhibited with the potent Y1 antagonist, BIBO 3304 (3 microg), but not the Y2 antagonist, BIIE 0246. When administered alone, BIBO 3304 (but not BIIE 0246) slightly decreased thermal PWL on the side ipsilateral (25% change), but not contralateral, to CFA injection; this suggests that inflammation strengthens inhibitory NPY tone. We conclude that spinal Y1 receptors contribute to the inhibitory effects of NPY on thermal hypersensitivity in the awake rat. Further studies are necessary to determine whether enhanced release of NPY and Y1-mediated inhibition of spinal nociceptive transmission ultimately results in a compensatory, adaptive inhibition of thermal hypersensitivity in the setting of inflammation.

NMDA or non-NMDA receptor antagonists attenuate increased Fos expression in spinal dorsal horn GABAergic neurons after intradermal injection of capsaicin in rats

GABAergic neurons play an important role in the generation of primary afferent depolarization, which results in presynaptic inhibition and, if large enough, triggers dorsal root reflexes. Recent electrophysiological studies by our group have suggested that increased excitation of spinal GABAergic neurons by activation of N-methyl-D-aspartate (NMDA) and non-NMDA receptors following intradermal injection of capsaicin results in the generation of DRRs that contribute to neurogenic inflammation. The present study was to determine if changes in the expression of Fos protein occur in GABAergic neurons in the lumbosacral spinal cord following injection of capsaicin into the glabrous skin of one hind paw of anesthetized rats and if pretreatment with an NMDA receptor antagonist, D-(-)-2-amino-7-phosphonoheptanoic acid (AP7) or a non-NMDA receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) blocks Fos expression in these neurons. The experiments used western blots and immunofluorescence double labeling staining following capsaicin or vehicle injection. Western blots showed that Fos protein was increased on the ipsilateral side in spinal cord tissue 0.5 h after capsaicin injection. Pretreatment with AP7 or CNQX caused a decrease in capsaicin-induced Fos expression. Immunofluorescence double labeling showed that the proportion of Fos-positive GABAergic neuronal profiles was significantly increased following capsaicin injection (48.8+/-4.8%) compared to the vehicle injection (23.8+/-5.1%) in superficial laminae on the ipsilateral side in lumbosacral spinal cord (P<0.05). However, when the spinal cord was pretreated with AP7 (5 microg) or CNQX (0.2 microg), only 9.1+/-0.6% or 7.1+/-0.8% of GABA-immunoreactive neuronal profiles were stained for Fos following capsaicin injection. The blockade of the capsaicin-evoked Fos staining was dose-dependent. These findings suggest that GABAergic neurons take part in dorsal horn circuits that modulate nociceptive information and that the function of GABAergic neurons following capsaicin injection is partially mediated by NMDA and non-NMDA receptors.

Glutamate uptake inhibition modulates dorsal horn neurotransmission: a comparison between normal and arthritic rats

This study determined the effects of glutamate uptake inhibition on primary-afferent excitatory postsynaptic potentials (DR-EPSPs) in spinal dorsal horn neurones in vitro from naive rats and rats with localised arthritis. The glutamate uptake inhibitor L-PDC (1 mM) significantly reduced DR-EPSP amplitude and duration with a greater reduction in arthritic than in naive rats. The group II/III selective metabotropic glutamate receptor antagonist CPPG (100 microM) reversed L-PDC-induced DR-EPSP inhibition in naive but not arthritic rats. L-AP4 (30 microM), a group III metabotropic agonist, inhibited DR-EPSPs with no difference between naive and arthritic rats. These data suggest the existence of an autoregulatory feedback mechanism that limits spinal postsynaptic excitation especially during inflammation. The putative contribution of metabotropic glutamate receptors to this phenomenon is discussed.

Presynaptic regulation of spinal cord tachykinin signaling via GABA(B) but not GABA(A) receptor activation

Internalization of spinal cord neurokinin-1 receptors following noxious stimulation provides a reliable measure of tachykinin signaling. In the present study, we examined the contribution of GABAergic mechanisms to the control of nociceptor processing involving tachykinins. Spinal administration of the GABA(B) receptor agonist R(+)-baclofen in the rat, at antinociceptive doses, significantly reduced the magnitude of neurokinin-1 receptor internalization in neurons of lamina I in response to acute noxious mechanical or thermal stimulation. By contrast, administration of even high doses of the GABA(A) receptor agonists, muscimol or isoguvacine, were without effect. CGP55845, a selective GABA(B) receptor antagonist, completely blocked the effects of baclofen, but failed to increase the incidence of internalization when administered alone. These results provide evidence for a presynaptic control of nociceptive primary afferent neurons by GABA(B) but not GABA(A) receptors in the superficial laminae of the spinal cord, limiting tachykinin release. Because CGP5584 alone did not increase the magnitude of neurokinin-1 receptor internalization observed following noxious stimulation, there appears to be little endogenous activation of GABA(B) receptors on tachykinin-releasing nociceptors under acute stimulus conditions. The contribution of pre- and postsynaptic regulatory mechanisms to GABA(B) receptor-mediated antinociception was also investigated by comparing the effect of baclofen on Fos expression evoked by noxious stimulation to that induced by intrathecal injection of substance P. In both instances, baclofen reduced Fos expression not only in neurons that express the neurokinin-1 receptor, but also in neurons that do not. We conclude that baclofen acts at presynaptic sites to reduce transmitter release from small-diameter nociceptive afferents. Presynaptic actions on non-tachykinin-containing nociceptors could similarly account for the reduction by baclofen of noxious stimulus-induced Fos expression in neurokinin-1 receptor-negative neurons. However, the inhibition of Fos expression induced by exogenous substance P indicates that actions at sites postsynaptic to tachykinin- and/or non-tachykinin-containing primary afferent terminals must also contribute to the antinociceptive actions of GABA(B) receptor agonists.

Activation of silent mechanoreceptive cat C and Adelta sensory neurons and their substance P expression following peripheral inflammation

The effect of inflammation on the excitability and the level of substance P (SP) in cat mechanoreceptive C and Adelta dorsal root ganglion (DRG) neurons were studied in vivo using intracellular recording and immunocytochemical techniques. Following injections of carrageenan (Carg) into the cat hindpaw, the percentage of C neurons exhibiting spontaneous activity increased from 7.2 to 20.7% and the percentage of Adelta neurons increased from 6.9 to 18.6%. In contrast to most cells from normal cats, which fired regularly below 10 Hz, many cells from Carg-treated cats fired at higher frequencies or in bursts. Inflammation (Carg treatment) also depolarized membrane potentials, increased membrane input resistance, caused the disappearance of inward rectifying currents and lowered the mean current thresholds of tibial nerve-evoked responses in DRG neurons. With inflammation, the percentage of C or Adelta neurons responding to low threshold mechanoreceptive stimuli increased (C neurons: normal, 13%; inflamed, 41%; Adelta neurons: normal, 13 %; inflamed, 39 %), while the percentage of C or Adelta neurons responding to high threshold mechanoreceptive stimuli remained unchanged. Some receptive field (RF)-responsive cells were injected with Lucifer Yellow and their SP immunoreactivity was determined. Following Carg treatment, substantially higher percentages of RF-responsive cells were SP positive (C neurons: normal, 35.7%; inflamed, 60%; Adelta neurons: normal, 18.2%; inflamed, 66.7%). These combined increases in the excitability of DRG neurons and SP-containing RF-responsive neurons could lead to sensitization of sensory neurons, thus contributing to the development of hyperalgesia.

Dorsal root reflexes and cutaneous neurogenic inflammation after intradermal injection of capsaicin in rats

The role of dorsal root reflexes (DRRs) in acute cutaneous neurogenic inflammation induced by intradermal injection of capsaicin (CAP) was examined in anesthetized rats. Changes in cutaneous blood flow (flare) on the plantar surface of the foot were measured using a laser Doppler flowmeter, and neurogenic edema was examined by measurements of paw thickness. To implicate DRRs in neurogenic inflammation after CAP injection, the ipsilateral sciatic and femoral nerves were sectioned, dorsal rhizotomies were performed at L(3-)-S(1), and antagonists of GABA or excitatory amino acid receptors were administered intrathecally. Intradermal injection of CAP evoked a flare response that was largest at 15-20 mm from the injection site and that spread >30 mm. Acute transection of the sciatic and femoral nerves or dorsal rhizotomies nearly completely abolished the blood flow changes 15-20 mm from the CAP injection site, although there was only a minimal effect on blood flow near the injection site. These procedures also significantly reduced neurogenic edema. Intrathecal bicuculline, 6-cyano-7-nitroquinoxaline-2,3-dione, (CNQX) or D(-)-2-amino-7-phosphonoheptanoic acid (AP7), but not phaclofen, also reduced dramatically the increases in blood flow 15-20 mm from the CAP injection site, but had only a minimal effect on blood flow near the injection site. Neurogenic edema was reduced by the same agents that reduced blood flow. Multiunit DRRs recorded from the central stumps of cut dorsal rootlets in the lumbar spinal cord were enhanced after CAP injection. This enhanced DRR activity could be reduced significantly by posttreatment of the spinal cord with bicuculline, CNQX or AP7, but not phaclofen. It is concluded that peripheral cutaneous inflammation induced by intradermal injection of CAP involves central nervous mechanisms. DRRs play a major role in the development of neurogenic cutaneous inflammation, although a direct action of CAP on peripheral nerve terminals or the generation of axon reflexes also may contribute to changes in the skin near the injection site.

Effects of GABA receptor antagonist on trigeminal caudalis nociceptive neurons in normal and neonatally capsaicin-treated rats

We have recently demonstrated that significant increases in cutaneous mechanoreceptive field (RF) size and spontaneous activity occur in nociceptive neurons of trigeminal subnucleus caudalis (Vc, the medullary dorsal horn) of adult rats depleted of C-fiber afferents by neonatal treatment with capsaicin. These neuronal changes in capsaicin-treated (CAP) rats are suggestive of central neuroplasticity and involve N-methyl-D-aspartic acid (NMDA) receptor mechanisms. The present study examined whether the GABA(A) receptor antagonist bicuculline (BIC) or the GABA(B) receptor antagonist 2-hydroxysaclofen (SAC) can influence the RF properties and activity of Vc nociceptive neurons classified as either nociceptive-specific or wide-dynamic range in CAP adult rats or in neonatally vehicle-treated (CON) rats. C-fiber depletion was confirmed in the CAP rats by a significant decrease in plasma extravasation of Evans blue dye in a skin area receiving topical application of mustard oil, a small-fiber excitant and inflammatory irritant. As previously reported, marked increases in cutaneous RF size and spontaneous activity occurred in Vc nociceptive neurons of adult CAP rats, compared with CON rats. GABA(A) receptor blockade by BIC (i.t.) in CON rats produced a significant increase in spontaneous activity and in pinch RF size and tactile RF size (or appearance of a tactile area in the RF of nociceptive-specific neurons), as well as a significant lowering of the mechanical threshold and a significant enhancement of responses to pinch stimuli applied to the RF. In CAP rats, GABA(A) receptor blockade also produced significant changes similar to those documented in CON rats, except for a paradoxical and significant decrease in pinch RF size and no noticeable changes in responses to pinch stimuli. GABA(B) receptor blockade by SAC (i.t. ) did not produce any significant changes in Vc nociceptive neurons in either CON or CAP rats. These results suggest that GABA(A) receptor-mediated inhibition may be involved in maintaining the functional expression of Vc nociceptive neuronal properties in normal conditions, and that in animals depleted of their C-fiber afferents, some features of this GABA(A) receptor-mediated modulation may be disrupted such that a GABA(A) receptor-mediated excitation is manifested.

Peripheral inflammation facilitates Abeta fiber-mediated synaptic input to the substantia gelatinosa of the adult rat spinal cord

Whole-cell patch-clamp recordings were made from substantia gelatinosa (SG) neurons in thick adult rat transverse spinal cord slices with attached dorsal roots to study changes in fast synaptic transmission induced by peripheral inflammation. In slices from naive rats, primary afferent stimulation at Abeta fiber intensity elicited polysynaptic EPSCs in only 14 of 57 (25%) SG neurons. In contrast, Abeta fiber stimulation evoked polysynaptic EPSCs in 39 of 62 (63%) SG neurons recorded in slices from rats inflamed by an intraplantar injection of complete Freund's adjuvant (CFA) 48 hr earlier (p < 0.001). Although the peripheral inflammation had no significant effect on the threshold and conduction velocities of Abeta, Adelta, and C fibers recorded in dorsal roots, the mean threshold intensity for eliciting EPSCs was significantly lower in cells recorded from rats with inflammation (naive: 33.2 +/- 15.1 microA, n = 57; inflamed: 22.8 +/- 11.3 microA, n = 62, p < 0.001), and the mean latency of EPSCs elicited by Abeta fiber stimulation in CFA-treated rats was significantly shorter than that recorded from naive rats (3.3 +/- 1.8 msec, n = 36 vs 6.0 +/- 3.5 msec, n = 12; p = 0.010). Abeta fiber stimulation evoked polysynaptic IPSCs in 4 of 25 (16%) cells recorded from naive rat preparations and 14 of 26 (54%) SG neurons from CFA-treated rats (p < 0.001). The mean threshold intensity for IPSCs was also significantly lower in CFA-treated rats (naive: 32.5 +/- 15.7 microA, n = 25; inflamed: 21. 9 +/- 9.9 microA, n = 26, p = 0.013). The facilitation of Abeta fiber-mediated input into the substantia gelatinosa after peripheral inflammation may contribute to altered sensory processing.

The induction of pain: an integrative review

The highly disagreeable sensation of pain results from an extraordinarily complex and interactive series of mechanisms integrated at all levels of the neuroaxis, from the periphery, via the dorsal horn to higher cerebral structures. Pain is usually elicited by the activation of specific nociceptors ('nociceptive pain'). However, it may also result from injury to sensory fibres, or from damage to the CNS itself ('neuropathic pain'). Although acute and subchronic, nociceptive pain fulfils a warning role, chronic and/or severe nociceptive and neuropathic pain is maladaptive. Recent years have seen a progressive unravelling of the neuroanatomical circuits and cellular mechanisms underlying the induction of pain. In addition to familiar inflammatory mediators, such as prostaglandins and bradykinin, potentially-important, pronociceptive roles have been proposed for a variety of 'exotic' species, including protons, ATP, cytokines, neurotrophins (growth factors) and nitric oxide. Further, both in the periphery and in the CNS, non-neuronal glial and immunecompetent cells have been shown to play a modulatory role in the response to inflammation and injury, and in processes modifying nociception. In the dorsal horn of the spinal cord, wherein the primary processing of nociceptive information occurs, N-methyl-D-aspartate receptors are activated by glutamate released from nocisponsive afferent fibres. Their activation plays a key role in the induction of neuronal sensitization, a process underlying prolonged painful states. In addition, upon peripheral nerve injury, a reduction of inhibitory interneurone tone in the dorsal horn exacerbates sensitized states and further enhance nociception. As concerns the transfer of nociceptive information to the brain, several pathways other than the classical spinothalamic tract are of importance: for example, the postsynaptic dorsal column pathway. In discussing the roles of supraspinal structures in pain sensation, differences between its 'discriminative-sensory' and 'affective-cognitive' dimensions should be emphasized. The purpose of the present article is to provide a global account of mechanisms involved in the induction of pain. Particular attention is focused on cellular aspects and on the consequences of peripheral nerve injury. In the first part of the review, neuronal pathways for the transmission of nociceptive information from peripheral nerve terminals to the dorsal horn, and therefrom to higher centres, are outlined. This neuronal framework is then exploited for a consideration of peripheral, spinal and supraspinal mechanisms involved in the induction of pain by stimulation of peripheral nociceptors, by peripheral nerve injury and by damage to the CNS itself. Finally, a hypothesis is forwarded that neurotrophins may play an important role in central, adaptive mechanisms modulating nociception. An improved understanding of the origins of pain should facilitate the development of novel strategies for its more effective treatment.

Relationship between neuronal activity and substance P-immunoreactivity in the rat spinal cord during acute and persistent myositis

The spinal level of substance P (SP) is assumed to be an important determinant of neuronal activity under pathophysiological conditions. In rat dorsal horn neurones, impulse activity was studied during a carrageenan-induced acute (2-8 h) and a Freund's adjuvant-induced persistent (12 days) myositis and compared with the spinal substance P-immunoreactivity (SP-IR) of the same animals. Myositis-induced changes in responsiveness of the neurones reached a maximum within 2-8 h, whereas background activity of the neurones was highest after 12 days of myositis. The area of SP-IR in the superficial dorsal horn decreased during acute and persistent myositis and the integrated density of the staining was largely unchanged. The difference in time-course between neuronal activity and SP-IR suggest that during persistent myositis factors other than SP gain more influence on the behaviour of the neurones.

Involvement of capsaicin sensitive primary afferents in thymulin-induced hyperalgesia

Intraplantar (5 ng) or intraperitoneal (50 ng) injections of thymulin, produced both thermal and mechanical hyperalgesia in rats. In this report, we show that ablation of capsaicin sensitive primary afferents (CSPA) can alter or abolish thymulin-induced hyperalgesia. Different groups of rats were subjected to either treatment with capsaicin or to surgical subdiaphragmatic vagotomy (SDV). Both capsaicin and SDV reduced significantly thymulin-induced hyperalgesia. On the other hand, these treatments elicited differential effects on the modulation by thymulin of the levels of nerve growth factor and interleukin 1beta. We conclude that the hyperalgesic effects of i.p. thymulin are mainly mediated through the CSPA fibers.

On the role of galanin in mediating spinal flexor reflex excitability in inflammation

The effects of exogenous and endogenous galanin on spinal flexor reflex excitability was evaluated in rats one to eight days after the induction of inflammation by subcutaneous injection of carrageenan into the sural nerve innervation area. In normal rats, electrical stimulation of C-fibres in the sural nerve elicited a brisk reflex discharge. Conditioning stimulation of C-fibres (1/s) generated a gradual increase in reflex magnitude (wind-up), which was followed by a period of reflex hyperexcitability. Intrathecal galanin dose-dependently blocked reflex hyperexcitability induced by C-fibre conditioning stimulation whereas i.t. M-35, a high-affinity galanin receptor antagonist, moderately potentiated this effect. At one to three days after the injection of carrageenen, when inflammation was at its peak, the magnitude of the reflex was significantly increased and discharge duration became prolonged. However, wind-up and reflex hyperexcitability were significantly reduced. Furthermore, reduced reflex excitability during conditioning stimulation ("wind-down") and depression of the reflex were sometimes present, which are rarely observed in normal rats. Intrathecal galanin reduced hyperexcitability during inflammation, although its potency was weaker than in normals. However, the galanin receptor antagonist M-35 strongly enhanced wind-up and reflex hyperexcitability, similarly as in normal rats. The baseline flexor reflex, wind-up and C-fibre conditioning stimulation-induced facilitation were normalized four to eight days after carrageenan injection when signs of inflammation were diminishing. Interestingly, intrathecal galanin and M-35 failed to influence spinal excitability. The results suggest a complex functional plasticity in the role of endogenous galanin in mediating spinal excitability during inflammation. There appears to be an enhanced endogenous inhibitory control by galanin on C-afferent input during the peak of inflammation, which may explain the relative ineffectiveness of exogenous galanin. During the recovery phase there may be a reduction in galanin receptors, which may impair the action of endogenous and exogenous galanin. These results further support the notion that galanin is an endogenous inhibitory peptide in nociception.

Sensitization of the nervous system, induced by two successive hindpaw inflammations, is suppressed by a local anesthetic

Two hindpaw injections of carrageenin were performed 7 days apart and the time-course of the vocalization thresholds to pressure (VTPP) of all the 4 paws of the rat were measured after both injections. The first injection of carrageenin induced an increase in the circumference of the injected paw and a significant reduction of the VTPPs of all the 4 paws. The pain-related behavior of both hindpaws was enhanced, when carrageenin was injected for the second time not only into the previously inflamed, but also into the contralateral hindpaw. However, the abnormal responses of the forepaws were not increased by this second hindpaw inflammation. Both after the first and the second injection of carrageenin, the decrease of the VTPPs of all 4 paws was antagonized by lidocaine with epinephrine (LE) injected into the inflamed paw. This anesthetic effect was of a shorter duration after the second than after the first injection of carrageenin. Presumably, the imprint that is left in the nervous system after the first hindpaw inflammation, takes place at a relatively limited segmental level. In addition, it is possible to influence the established sensitization of the nervous system by treating the peripheral process itself, even when the first injury has primed the nervous system to the second injury. However, there still seems to exist some excitatory influences that cannot be suppressed by the local anesthetic.

The pharmacology of excitatory and inhibitory amino acid-mediated events in the transmission and modulation of pain in the spinal cord

1. The aim of this review is to consider the relative roles of inhibitory and excitatory amino acid receptor-mediated events in the processes leading to pain transmission in the spinal cord. 2. Emphasis will be on the roles of the inhibitory and excitatory amino acids, GABA and glutamate, and how the relative balance between activity in these systems appears to determine the level of pain transmission. 3. The N-methyl-D-aspartate (NMDA) receptor for glutamate has been implicated in the generation and maintenance of central (spinal) states of hypersensitivity. It has been shown that activation of this receptor underlies wind-up, whereby the level of transmission of noxious messages is potentiated. Antagonists at this receptor-channel complex prevent or block enhanced (hyperalgesic) pain states induced by tissue damage, inflammation, nerve damage and ischemia. 4. Information concerning amplification systems in the spinal cord, such as the NMDA receptor, is a step toward understanding why and how a painful response is not always matched to the stimulus. Such events have parallels with other plastic events such as long-term potentiation (LTP) in the hippocampus. 5. However, the roles of inhibitory transmitter systems can also change insofar as opioid, adenosine and GABA transmission in the spinal cord can vary in different pain states. 6. Changes in GABA systems have been well-documented and discussion will center on whether this has clinical implications. 7. In addition to behavioral and electrophysiological approaches to the pharmacology of pain the current status of the use of markers of early onset genes such as c-fos, as monitors of activity, will be discussed. 8. Hyperalgesia would appear to be balanced by inhibitions during inflammatory conditions but not in neuropathic states, pains due to nerve damage. In the latter case, events reminiscent of LTP may predominate, whereas they are held in check by inhibitions under conditions of inflammation.

Loss of GABA-immunoreactivity in the spinal dorsal horn of rats with peripheral nerve injury and promotion of recovery by adrenal medullary grafts

Abnormal pain-related behaviour that accompanies peripheral nerve injury may be the result of altered spinal neuronal function. The long-term loss of inhibitory function by GABA neurons in particular may be a mechanism by which abnormal neural hyperactivity occurs, leading to exaggerated sensory processing following nerve injury. In order to assess this, changes in spinal GABA immunoreactivity at several time points following constriction nerve injury were quantified in parallel with behavioural assessments of abnormal sensory responses to noxious and innocuous stimuli. In addition, the effects of spinal adrenal medullary transplants were determined since previous findings have demonstrated alleviation of behavioural pain symptoms by such transplants. In response to unilateral sciatic nerve injury, GABAergic profiles normally found in lumbar dorsal horn laminae I-III significantly decreased. The decrease was apparent three days following ligation, particularly on the side ipsilateral to the nerve injury. By two weeks, no GABAergic profiles could be seen, with the deficit appearing in the spinal dorsal horn both ipsilateral and contralateral to the unilateral peripheral nerve injury. Marked decreases in GABA-immunoreactive profiles persisted for at least up to five weeks post-injury, with partial restoration occurring by seven weeks. However, even at seven weeks, losses in GABA-immunoreactive profiles persisted in the dorsal horn ipsilateral to peripheral nerve injury. These findings were comparable in animals receiving control striated muscle transplants. In contrast, adrenal medullary transplants markedly reduced the loss in GABA-immunoreactive profiles at all time-points examined. In addition, GABA-immunoreactive profile levels were normalized near that of intact animals by five to seven weeks following nerve injury in animals with adrenal medullary transplants. Parallel improvements in sensory responses to innocuous and noxious stimuli were also observed in these animals. The results of this study indicate that peripheral nerve injury can result in severe losses in spinal inhibitory mechanisms, possibly leading to exaggerated sensory processes in persistent pain states. In addition, adrenal medullary transplants may provide a neuroprotective function in promoting recovery and improving long-term survival of GABAergic neurons in the spinal dorsal horn which have been damaged by excitotoxic injury.

Mechanisms of touch-evoked pain (allodynia): a new model

In this paper we review the current neurophysiological models of touch-evoked pain and present a new proposal that addresses the mechanisms of allodynia. The new model is based on the notion that A-beta mechanoreceptors can gain access to nociceptive neurones by means of a presynaptic link, at central level, between low threshold mechanoreceptors and nociceptors. We propose that the excitation of nociceptors provoked by a peripheral injury activates the spinal interneurones that mediate primary afferent depolarization (PAD) between low threshold mechanoreceptors and nociceptors. As a consequence of the increased and persistent barrage driving these neurones their excitability is increased such that, when activated by low threshold mechanoreceptors from areas surrounding the injury site, they produce a very intense PAD in the nociceptive afferents which is capable of generating spike activity. This activation would be conducted antidromically in the form of dorsal root reflexes (DRRs) but would also be conducted forward activating the second order neurones normally driven by nociceptors. The sensory consequence of this mechanism is pain evoked by the activation of low threshold mechanoreceptors from an area surrounding an injury site (allodynia).

Pain mechanisms involved in musculoskeletal disorders

This manuscript is intended to give a basic review of the peripheral and spinal neuronal mechanisms involved in the processing of musculoskeletal pain. There is a complicated neuronal network in the periphery and the spinal cord for the processing of nociceptive information. Injury to a muscle (inflammation or ischemia) or a joint (inflammation) results in sensitization of peripheral nociceptors. There is then an increased transmission to and increased release of neurotransmitters in the dorsal horn of the spinal cord. Dorsal horn neurons sensitized by the peripheral injury demonstrate increased background activity, increased receptive field size, and increased responses to peripherally applied stimuli. The increased release of neurotransmitters and the sensitization of dorsal horn neurons is dependent on activation of N-methyl-D-aspartate (NMDA), non-NMDA excitatory amino acid, and neurokinin 1 receptors. Behavioral changes typical of inflammatory pain are observed in arthritic rats. These behavioral changes can be modified by a variety of drugs, including opioids, excitatory amino acid receptor antagonists, or neurokinin receptor antagonists. In addition to processing nociceptive information following joint or muscle injury, the spinal cord controls peripheral joint inflammation. Production of dorsal root reflexes, antidromic action potentials, would be expected to result in the release of inflammatory neuropeptides [substance P and calcitonin gene-related peptide (CGRP)] from the terminals of primary afferents at the site of injury. The release of substance P and CGRP would potentiate the inflammatory response in the periphery.

The contribution of GABAB receptor-mediated events to inflammatory pain processing: carrageenan oedema and associated spinal c-Fos expression in the rat

In this pharmacological study we have assessed the effect of baclofen, a selective GABAB receptor agonist, on spinal expression of the immediate early gene c-Fos and the peripheral oedema evoked by a prolonged peripheral inflammation due to intraplantar carrageenan. Baclofen was administered intravenously 30 min before intraplantar injection of carrageenan in freely moving rats. Three hours after carrageenan the number of spinal c-Fos protein-like immunoreactive neurons and peripheral (ankle and paw) oedema were assessed. For the two series of experiments the total number of control carrageenan-evoked c-Fos protein-like immunoreactive neurons in segments L4-L5 of the spinal cord was 176 +/- 6 and 177 +/- 9 c-Fos protein-like immunoreactive neurons per section, for carrageenan control with intravenous and intraplantar saline, respectively. c-Fos protein-like immunoreactive neurons were predominantly located in laminae I-II and V-VI of the dorsal horn of the spinal cord in carrageenan controls receiving intravenous (68 +/- 3 and 69 +/- 2 c-Fos protein-like immunoreactive neurons, respectively) and intraplantar (62 +/- 4 and 71 +/- 5 c-Fos protein-like immunoreactive neurons, respectively) saline. Pre-administered systemic baclofen (0.05, 1.5 and 3 mg/kg i.v.) dose dependently reduced the total number of c-Fos protein-like immunoreactive neurons (81 +/- 3, 66 +/- 4 and 49 +/- 4% of control total number of c-Fos protein-like immunoreactive neurons, respectively), with strongest effects on the number of deep (74 +/- 3, 60 +/- 3 and 43 +/- 4% of control, respectively) as compared with superficial (90 +/- 4, 77 +/- 5 and 59 +/- 5% of control, respectively) c-Fos protein-like immunoreactive neurons. The effects of systemic baclofen on the carrageenan-induced spinal c-Fos expression and both the paw and ankle oedema were positively correlated (r = 0.479, P < 0.05 and r = 0.733, P < 0.001, respectively). Intraplantar baclofen (50 and 100 micrograms in 50 microliters of saline), simultaneously injected with intraplantar carrageenan, did not significantly influence carrageenan-evoked spinal c-Fos expression or ankle oedema. Despite the fact that the highest dose of intraplantar baclofen significantly reduced paw oedema (23 +/- 3% reduction of control paw oedema), our results are clearly in favour of a spinal site of action of systemic baclofen.

Blockade of capsaicin-induced reduction of GABA-immunoreactivity by spantide in cat spinal superficial dorsal horn

In our previous study, perineural application of capsaicin not only produced release and depletion of substance P from primary nociceptive afferent terminals, but also reduced GABA immunoreactivity in the superficial dorsal horn. The aim of the present study was to determine whether the release of GABA is triggered by substance P released from primary nociceptive afferent terminals by capsaican. GABA and substance P immunoreactivity in the lumbar dorsal horn was examined in two groups: in the first group the tibial nerve was treated with 3% capsaicin, and in the second group the dorsal surface of the lumbar cord was infused with spantide (50 nM), a substance P receptor antagonist, before application of capsaicin to the tibial nerve. Following perineural treatment of capsaicin for 30 min, both the GABA-immunostaining density and the number of GABA immunoreactive neurons were reduced significantly in the ipsilateral laminae I-II at L5 through L7. GABA immunoreactivity was reduced by 54.12%, 44.46% and 31.0% in the medial, central and lateral parts of the ipsilateral laminae I-II at L7, respectively. With pre-application of spantide to the spinal cord, GABA immunoreactivity was reduced only to 14.4%, 16.4% and 10.16%, respectively, in the medial, central and lateral parts of laminae I-II at L7 and no reduction of GABA immunoreactive neurons was observed. Additionally, capsaicin-induced reduction of substance P immunoreactivity was partially blocked by spantide. These results suggest that capsaicin produces substance P release from primary nociceptive afferent terminals, and that substance P, in turn, activates the second-order GABAergic interneurons in the dorsal horn. The functional significance of capsaicin-induced activation of GABAergic neurons in modulation of spinal nociception is discussed.