Roles of monoaminergic, glycinergic and GABAergic inhibitory systems in the spinal cord in rats with peripheral mononeuropathy

Centralizing the Knowledge and Interpretation of Pain in Chemotherapy-Induced Peripheral Neuropathy: A Paradigm Shift towards Brain-Centric Approaches

Chemotherapy-induced peripheral neuropathy (CIPN) is a side effect of cancer treatment, often linked with pain complaints. Patients report mechanical and thermal hypersensitivity that may emerge during chemotherapy treatment and may persist after cancer remission. Whereas the latter situation disturbs the quality of life, life itself may be endangered by the appearance of CIPN during cancer treatment. The causes of CIPN have almost entirely been ascribed to the neurotoxicity of chemotherapeutic drugs in the peripheral nervous system. However, the central consequences of peripheral neuropathy are starting to be unraveled, namely in the supraspinal pain modulatory system. Based on our interests and experience in the field, we undertook a review of the brain-centered alterations that may underpin pain in CIPN. The changes in the descending pain modulation in CIPN models along with the functional and connectivity abnormalities in the brain of CIPN patients are analyzed. A translational analysis of preclinical findings about descending pain regulation during CIPN is reviewed considering the main neurochemical systems (serotoninergic and noradrenergic) targeted in CIPN management in patients, namely by antidepressants. In conclusion, this review highlights the importance of studying supraspinal areas involved in descending pain modulation to understand the pathophysiology of CIPN, which will probably allow a more personalized and effective CIPN treatment in the future.

Monoaminergic and Opioidergic Modulation of Brainstem Circuits: New Insights Into the Clinical Challenges of Pain Treatment?

The treatment of neuropathic pain remains a clinical challenge. Analgesic drugs and antidepressants are frequently ineffective, and opioids may induce side effects, including hyperalgesia. Recent results on brainstem pain modulatory circuits may explain those clinical challenges. The dual action of noradrenergic (NA) modulation was demonstrated in animal models of neuropathic pain. Besides the well-established antinociception due to spinal effects, the NA system may induce pronociception by directly acting on brainstem pain modulatory circuits, namely, at the locus coeruleus (LC) and medullary dorsal reticular nucleus (DRt). The serotoninergic system also has a dual action depending on the targeted spinal receptor, with an exacerbated activity of the excitatory 5-hydroxytryptamine 3 (5-HT3) receptors in neuropathic pain models. Opioids are involved in the modulation of descending modulatory circuits. During neuropathic pain, the opioidergic modulation of brainstem pain control areas is altered, with the release of enhanced local opioids along with reduced expression and desensitization of μ-opioid receptors (MOR). In the DRt, the installation of neuropathic pain increases the levels of enkephalins (ENKs) and induces desensitization of MOR, which may enhance descending facilitation (DF) from the DRt and impact the efficacy of exogenous opioids. On the whole, the data discussed in this review indicate the high plasticity of brainstem pain control circuits involving monoaminergic and opioidergic control. The data from studies of these neurochemical systems in neuropathic models indicate the importance of designing drugs that target multiple neurochemical systems, namely, maximizing the antinociceptive effects of antidepressants that inhibit the reuptake of serotonin and noradrenaline and preventing desensitization and tolerance of MOR at the brainstem.

A systematic review on descending serotonergic projections and modulation of spinal nociception in chronic neuropathic pain and after spinal cord stimulation

Chronic neuropathic pain is a debilitating ordeal for patients worldwide and pharmacological treatment efficacy is still limited. As many pharmacological interventions for neuropathic pain often fail, insights into the underlying mechanism and role of identified receptors is of utmost importance. An important target for improving treatment of neuropathic pain is the descending serotonergic system as these projections modulate nociceptive signaling in the dorsal horn. Also with use of last resort treatments like spinal cord stimulation (SCS), the descending serotonergic projections are known to be involved in the pain relieving effect. This systematic review summarizes the involvement of the serotonergic system on nociceptive modulation in the healthy adult rodent and the chronic neuropathic rodent and summarizes all available literature on the serotonergic system in the SCS-treated neuropathic rodent. Medline, Embase and Pubmed databases were used in the search for articles. Descending serotonergic modulation of nociceptive signaling in spinal dorsal horn in normal adult rat is mainly inhibitory and mediated by 5-HT1a, 5-HT1b, 5-HT2c, 5-HT3 and 5-HT4 receptors. Upon injury and in the neuropathic rat, this descending serotonergic modulation becomes facilitatory via activation of the 5-HT2a, 5-HT2b and 5-HT3 receptors. Analgesia due to neuromodulatory intervention like SCS restores the inhibitory function of the descending serotonergic system and involves 5-HT2, 5-HT3 and 5-HT4 receptors. The results of this systematic review provide insights and suggestions for further pharmacological and or neuromodulatory treatment of neuropathic pain based on targeting selected serotonergic receptors related to descending modulation of nociceptive signaling in spinal dorsal horn. With the novel developed SCS paradigms, the descending serotonergic system will be an important target for mechanism-based stimulation induced analgesia.

Enhancing neuronal chloride extrusion rescues α2/α3 GABAA-mediated analgesia in neuropathic pain

Spinal disinhibition has been hypothesized to underlie pain hypersensitivity in neuropathic pain. Apparently contradictory mechanisms have been reported, raising questions on the best target to produce analgesia. Here, we show that nerve injury is associated with a reduction in the number of inhibitory synapses in the spinal dorsal horn. Paradoxically, this is accompanied by a BDNF-TrkB-mediated upregulation of synaptic GABAARs and by an α1-to-α2GABAAR subunit switch, providing a mechanistic rationale for the analgesic action of the α2,3GABAAR benzodiazepine-site ligand L838,417 after nerve injury. Yet, we demonstrate that impaired Cl- extrusion underlies the failure of L838,417 to induce analgesia at high doses due to a resulting collapse in Cl- gradient, dramatically limiting the benzodiazepine therapeutic window. In turn, enhancing KCC2 activity not only potentiated L838,417-induced analgesia, it rescued its analgesic potential at high doses, revealing a novel strategy for analgesia in pathological pain, by combined targeting of the appropriate GABAAR-subtypes and restoring Cl- homeostasis.

Reviewing the case for compromised spinal inhibition in neuropathic pain

A striking and debilitating property of the nervous system is that damage to this tissue can cause chronic intractable pain, which persists long after resolution of the initial insult. This neuropathic form of pain can arise from trauma to peripheral nerves, the spinal cord, or brain. It can also result from neuropathies associated with disease states such as diabetes, human immunodeficiency virus/AIDS, herpes, multiple sclerosis, cancer, and chemotherapy. Regardless of the origin, treatments for neuropathic pain remain inadequate. This continues to drive research into the underlying mechanisms. While the literature shows that dysfunction in numerous loci throughout the CNS can contribute to chronic pain, the spinal cord and in particular inhibitory signalling in this region have remained major research areas. This review focuses on local spinal inhibition provided by dorsal horn interneurons, and how such inhibition is disrupted during the development and maintenance of neuropathic pain.

Association of Serum Serotonin and Pain in Patients with Chronic Low Back Pain before and after Spinal Surgery

Introduction

In this study we are aiming to evaluate the changes of serum serotonin and its association with pain in patients suffering from chronic low back pain before and after lumbar discectomy surgery.

Patients and Methods

A prospective study was performed on the patients referring to the outpatient clinic in Besat hospital, Hamadan University of Medical Sciences, Hamadan, Iran, during 2016. A 2 mL fasting blood sample was collected from each patient at preoperative day 1 and postoperative day 14 and they were measured for level of serum serotonin. Besides, all patients were asked for severity of their low back pain in preoperative day 1 and postoperative day 14 and scored their pain from zero to ten using a Numerical Rating Scale.

Results

Forty patients with the mean age of 47 ± 13 yrs/old (range 25–77) including 15 (37.5%) males were enrolled into the study. The overall mean score of preoperative pain was significantly decreased from 7.4 ± 2.18 (range 4–10) to the postoperative pain score 3.87 ± 2.92 (range 0–10) (P < .001). The overall levels of pre- and postoperative serum serotonin were 3.37 ± 1.27 (range 1.1–6.4) and 3.58 ± 1.32 (range .94–7.1) ng/mL, respectively, with no significant difference (P = .09). The levels of pre- and postoperative serum serotonin were significantly higher in males and patients older than 50 yrs/old compared to the females and patients younger than 50 yrs/old, respectively (P = .03 and .005, respectively). A significant inverse correlation between the postoperative levels of pain and serum serotonin was observed (r = -.36 and P = .02).

Conclusion

A negative medium strength linear relationship may exist between the postoperative serum serotonin and low back pain.

Regular physical activity prevents development of chronic muscle pain through modulation of supraspinal opioid and serotonergic mechanisms

The current study shows that blockade of opioid receptors systemically in the periaqueductal gray and the rostral ventromedial medulla prevents analgesia by 8 weeks of wheel running in a chronic muscle pain model. We further show increases in serotonin transporter expression and reversal of hyperalgesia with a selective reuptake inhibitor in the rostral ventromedial medulla in the chronic muscle pain model, and exercise normalizes serotonin transporter expression.

Introduction:

It is generally believed that exercise produces its effects by activating central opioid receptors; there are little data that support this claim. The periaqueductal gray (PAG) and rostral ventromedial medulla (RVM) are key nuclei in opioid-induced analgesia, and opioids interact with serotonin to produce analgesia.

Objectives:

The purpose was to examine central inhibitory mechanisms involved in analgesia produced by wheel running.

Methods:

C57/Black6 mice were given access to running wheels in their home cages before induction of chronic muscle hyperalgesia and compared with those without running wheels. Systemic, intra-PAG, and intra-RVM naloxone tested the role of central opioid receptors in the antinociceptive effects of wheel running in animals with muscle insult. Immunohistochemistry for the serotonin transporter (SERT) in the spinal cord and RVM, and pharmacological blockade of SERT, tested whether the serotonin system was modulated by muscle insult and wheel running.

Results:

Wheel running prevented the development of muscle hyperalgesia. Systemic naloxone, intra-PAG naloxone, and intra-RVM naloxone reversed the antinociceptive effect of wheel running in animals that had received muscle insult. Induction of chronic muscle hyperalgesia increased SERT in the RVM, and blockade of SERT reversed the hyperalgesia in sedentary animals. Wheel running reduced SERT expression in animals with muscle insult. The serotonin transporter in the superficial dorsal horn of the spinal cord was unchanged after muscle insult, but increased after wheel running.

Conclusion:

These data support the hypothesis that wheel running produced analgesia through central inhibitory mechanisms involving opioidergic and serotonergic systems.

Modulation of Spinal GABAergic Inhibition and Mechanical Hypersensitivity following Chronic Compression of Dorsal Root Ganglion in the Rat

Chronic compression of dorsal root ganglion (CCD) results in neuropathic pain. We investigated the role of spinal GABA in CCD-induced pain using rats with unilateral CCD. A stereological analysis revealed that the proportion of GABA-immunoreactive neurons to total neurons at L4/5 laminae I-III on the injured side decreased in the early phase of CCD (post-CCD week 1) and then returned to the sham-control level in the late phase (post-CCD week 18). In the early phase, the rats showed an increase in both mechanical sensitivity of the hind paw and spinal WDR neuronal excitability on the injured side, and such increase was suppressed by spinally applied muscimol (GABA-A agonist, 5 nmol) and baclofen (GABA-B agonist, 25 nmol), indicating the reduced spinal GABAergic inhibition involved. In the late phase, the CCD-induced increase in mechanical sensitivity and neuronal excitability returned to pre-CCD levels, and such recovered responses were enhanced by spinally applied bicuculline (GABA-A antagonist, 15 nmol) and CGP52432 (GABA-B antagonist, 15 nmol), indicating the regained spinal GABAergic inhibition involved. In conclusion, the alteration of spinal GABAergic inhibition following CCD and leading to a gradual reduction over time of CCD-induced mechanical hypersensitivity is most likely due to changes in GABA content in spinal GABA neurons.

Colokinetic effect of noradrenaline in the spinal defecation center: implication for motility disorders

Chronic abdominal pain in irritable bowel syndrome (IBS) usually appears in combination with disturbed bowel habits, but the etiological relationship between these symptoms remains unclear. Noradrenaline is a major neurotransmitter controlling pain sensation in the spinal cord. To test the hypothesis that the descending noradrenergic pathway from the brain stem moderates gut motility, we examined effects of intrathecal application of noradrenaline to the spinal defecation center on colorectal motility. Colorectal intraluminal pressure and expelled volume were recorded in vivo in anesthetized rats. Intrathecal application of noradrenaline into the L6-S1 spinal cord, where the lumbosacral defecation center is located, caused propulsive contractions of the colorectum. Inactivation of spinal neurons by tetrodotoxin blocked the effect of noradrenaline. Pharmacological experiments showed that the effect of noradrenaline is mediated primarily by alpha-1 adrenoceptors. The enhancement of colorectal motility by intrathecal noradrenaline was abolished by severing of the pelvic nerves. Our results demonstrate that noradrenaline acting on sacral parasympathetic preganglionic neurons through alpha-1 adrenoceptors causes propulsive motility of the colorectum in rats. Considering that visceral pain activates the descending inhibitory pathways including noradrenergic neurons, our results provide a rational explanation of the concurrent appearance of chronic abdominal pain and colonic motility disorders in IBS patients.

Effect of antioxidant treatment on spinal GABA neurons in a neuropathic pain model in the mouse

One feature of neuropathic pain is a reduced spinal gamma-aminobutyric acid (GABA)-ergic inhibitory function. However, the mechanisms behind this attenuation remain to be elucidated. This study investigated the involvement of reactive oxygen species in the spinal GABA neuron loss and reduced GABA neuron excitability in spinal nerve ligation (SNL) model of neuropathic pain in mice. The importance of spinal GABAergic inhibition in neuropathic pain was tested by examining the effects of intrathecally administered GABA receptor agonists and antagonists in SNL and naïve mice, respectively. The effects of SNL and antioxidant treatment on GABA neuron loss and functional changes were examined in transgenic GAD67-enhanced green fluorescent protein positive (EGFP+) mice. GABA receptor agonists transiently reversed mechanical hypersensitivity of the hind paw in SNL mice. On the other hand, GABA receptor antagonists made naïve mice mechanically hypersensitive. Stereological analysis showed that the numbers of enhanced green fluorescent protein positive (EGFP+) GABA neurons were significantly decreased in the lateral superficial laminae (I-II) on the ipsilateral L5 spinal cord after SNL. Repeated antioxidant treatments significantly reduced the pain behaviors and prevented the reduction in EGFP+ GABA neurons. The response rate of the tonic firing GABA neurons recorded from SNL mice increased with antioxidant treatment, whereas no change was seen in those recorded from naïve mice, which suggested that oxidative stress impaired some spinal GABA neuron activity in the neuropathic pain condition. Together the data suggest that neuropathic pain, at least partially, is attributed to oxidative stress, which induces both a GABA neuron loss and dysfunction of surviving GABA neurons.

Peripheral neuropathy induces cutaneous hypersensitivity in chronically spinalized rats

BACKGROUND/OBJECTIVES

The present study was aimed at the issue of whether peripheral nerve injury-induced chronic pain is maintained by supraspinal structures governing descending facilitation to the spinal dorsal horn, or whether altered peripheral nociceptive mechanisms sustain central hyperexcitability and, in turn, neuropathic pain. We examined this question by determining the contribution of peripheral/spinal mechanisms, isolated from supraspinal influence(s), in cutaneous hypersensitivity in an animal model of peripheral neuropathy.

METHODS

Adult rats were spinalized at T8-T9; 8 days later, peripheral neuropathy was induced by implanting a 2-mm polyethylene cuff around the left sciatic nerve. Hind paw withdrawal responses to mechanical or thermal plantar stimulation were evaluated using von Frey filaments or a heat lamp, respectively.

RESULTS

Spinalized rats without cuff implantation exhibited a moderate decrease in mechanical withdrawal threshold on ~day 10 (P < 0.05) and in thermal withdrawal threshold on ~day 18 (P < 0.05). However, cuff-implanted spinalized rats developed a more rapid and significant decrease in mechanical (~day 4; P < 0.001) and thermal (~day 10; P < 0.05) withdrawal thresholds that remained significantly decreased through the duration of the study.

CONCLUSIONS

Our findings demonstrate an aberrant peripheral/spinal mechanism that induces and maintains thermal and to a greater degree tactile cutaneous hypersensitivity in the cuff model of neuropathic pain, and raise the prospect that altered peripheral/spinal nociceptive mechanisms in humans with peripheral neuropathy may have a pathologically relevant role in both inducing and sustaining neuropathic pain.

An increase in spinal cord noradrenaline is a major contributor to the antihyperalgesic effect of antidepressants after peripheral nerve injury in the rat

Antidepressants are often used for the treatment of neuropathic pain. Clinical studies suggest that the efficacy of serotonin (5-HT) and noradrenaline (NA) reuptake inhibitors (SNRIs) for neuropathic pain is greater than that of selective 5-HT reuptake inhibitors (SSRIs). In the present study, we determined the efficacy and mechanisms involved in the antihyperalgesic effects of milnacipran, an SNRI, compared with paroxetine, an SSRI, and maprotiline, a selective NA reuptake inhibitor, using a rat model of neuropathic pain. Male Sprague-Dawley rats underwent spinal nerve ligation (SNL), and the withdrawal threshold to paw pressure was measured. Intraperitoneal injection of milnacipran (3-30mg/kg) produced a dose-dependent antihyperalgesic effect. The effect was reversed by intrathecal injection of the α(2)-adrenoceptor antagonist idazoxan (30μg), but not by various 5-HT receptor antagonists. Paroxetine produced an antihyperalgesic effect only at the highest dose tested (10mg/kg). This effect was reversed by intrathecal injection of both idazoxan and ondansetron (30μg), a 5-HT3 receptor antagonist. Maprotiline produced an antihyperalgesic effect (10 and 30mg/kg), and the effect was reversed by intrathecal idazoxan. In microdialysis studies, NA and 5-HT concentrations in the spinal dorsal horn were increased after injection of either milnacipran or paroxetine, and only NA was increased after maprotiline. Furthermore, the NA content in the spinal cord of SNL rats was greater than that in normal animals. These findings suggest that an increase in NA in the spinal cord plays an important role in the antihyperalgesic effects of not only NA reuptake inhibitors but also SSRIs.

Differential GABAergic disinhibition during the development of painful peripheral neuropathy

An impaired spinal GABAergic inhibitory function is known to be pivotal in neuropathic pain (NPP). At present, data concerning time-dependent alterations within the GABAergic system itself and post-synaptic GABA(A) receptor-mediated inhibitory transmission are highly controversial, likely related to the experimental NPP model used. Furthermore, it is unknown whether the severity of NPP is determined by the degree of these GABAergic disturbances. In the present study we therefore examined in one experimental animal model whether anatomical changes within the spinal GABAergic system and its GABA(A) receptor-mediated inhibitory function are gradually aggravated during the development of partial sciatic nerve injury (PSNL)-induced NPP and are related to the severity of PSNL-induced hypersensitivity. Three and 16 days after a unilateral PSNL (early and late NPP, respectively), GABA-immunoreactivity (GABA-IR) and the number of GABA-IR neuronal profiles were determined in Rexed laminae 1-3 of lumbar spinal cord cryosections. Additionally, the efficiency of dorsal horn GABA(A) receptor-induced inhibition was examined by cation chloride cotransporter 2 (KCC2) immunoblotting. NPP-induced hypersensitivity was only observed at the ipsilateral side, both at early and late time points. During early NPP, a decrease in ipsilateral dorsal horn GABA-IR was observed without alterations in the number of GABA-IR neuronal profiles or KCC2 protein levels. In contrast, bilateral increases in spinal GABA-IR accompanied by an unchanged number of GABA-IR interneurons were observed during late NPP. This was furthermore attended with decreased ipsilateral KCC2 levels. Moreover, the degree of hypersensitivity was not related to disturbances within the spinal GABAergic system at all time points examined. In conclusion, our anatomical data suggest that a dysfunctional GABA production is likely to be involved in early NPP whereas late NPP is characterized by a combined dysfunctional GABA release and decreased KCC2 levels, the latter suggesting an impaired GABA(A) receptor-mediated inhibition.

Lack of analgesic efficacy of spinal ondansetron on thermal and mechanical hypersensitivity following spinal nerve ligation in the rat

The balance between descending inhibition and facilitation is thought to be disturbed in chronic pain states. Increased facilitation by spinally released serotonin has been suggested by demonstration that mechanically evoked neuronal responses of wide dynamic range neurons are inhibited by 5-HT3 receptor antagonists in rats following spinal nerve ligation (SNL) but not sham operation. Despite these physiologic data, the effects of spinal 5-HT3 receptor blockade on behavioral hypersensitivity and neurochemical alterations in spinal serotonergic system have not been thoroughly investigated following spinal nerve ligation in the rat. To test this, we acutely injected intrathecal ondansetron in rats between 14 and 30 days after SNL and assessed effects on thermal and mechanical hypersensitivity. We also determined the density of serotonergic nerve fibers, serotonin content and the levels of 5-HT3 receptors within the spinal cord at this time point. Intrathecal ondansetron (1, 3, 10, 30, and 100microg) produced no effect on behavioral measures of thermal or mechanical hypersensitivity whereas intrathecal morphine (1microg) and gabapentin (200microg) partially reversed thermal and mechanical hypersensitivity following SNL. In addition, SNL did not alter the density of serotonergic fibers or 5-HT3 receptor immunoreactivity or spinal tissue content of 5-HT within the dorsal horn. These results do not support anatomic plasticity of descending serotonergic pathways or tonic 5-HT3 receptor activity in maintaining hypersensitivity after nerve injury and in contrast to previous studies fail to demonstrate an anti-hypersensitivity effect of intrathecal injection of the 5-HT3 receptor antagonist ondansetron. Importantly, behavioral measures of mechanical hypersensitivity assess threshold responses whereas physiological studies of mechanically evoked neuronal responses involve application of suprathreshold stimuli. Thus, suprathreshold or more intense stimuli may be necessary to recruit descending serotonergic facilitatory drive required to observe the inhibitory effects of ondansetron on spinal neuronal excitability and behavioral hypersensitivity.

Are spinal GABAergic elements related to the manifestation of neuropathic pain in rat?

Impairment in spinal inhibition caused by quantitative alteration of GABAergic elements following peripheral nerve injury has been postulated to mediate neuropathic pain. In the present study, we tested whether neuropathic pain could be induced or reversed by pharmacologically modulating spinal GABAergic activity, and whether quantitative alteration of spinal GABAergic elements after peripheral nerve injury was related to the impairment of GABAergic inhibition or neuropathic pain. To these aims, we first analyzed the pain behaviors following the spinal administration of GABA antagonists (1 microg bicuculline/rat and 5 microg phaclofen/rat), agonists (1 microg muscimol/rat and 0.5 microg baclofen/rat) or GABA transporter (GAT) inhibitors (20 microg NNC-711/rat and 1 microg SNAP-5114/rat) into naïve or neuropathic animals. Then, using Western blotting, PCR or immunohistochemistry, we compared the quantities of spinal GABA, its synthesizing enzymes (GAD65, 67) and its receptors (GABA(A) and GABA(B)) and transporters (GAT-1, and -3) between two groups of rats with different severity of neuropathic pain following partial injury of tail-innervating nerves; the allodynic and non-allodynic groups. Intrathecal administration of GABA antagonists markedly lowered tail-withdrawal threshold in naïve animals, and GABA agonists or GAT inhibitors significantly attenuated neuropathic pain in nerve-injured animals. However, any quantitative changes in spinal GABAergic elements were not observed in both the allodynic and non-allodynic groups. These results suggest that although the impairment in spinal GABAergic inhibition may play a role in mediation of neuropathic pain, it is not accomplished by the quantitative change in spinal elements for GABAergic inhibition and therefore these elements are not related to the generation of neuropathic pain following peripheral nerve injury.

Models and mechanisms of hyperalgesia and allodynia

Hyperalgesia and allodynia are frequent symptoms of disease and may be useful adaptations to protect vulnerable tissues. Both may, however, also emerge as diseases in their own right. Considerable progress has been made in developing clinically relevant animal models for identifying the most significant underlying mechanisms. This review deals with experimental models that are currently used to measure (sect. II) or to induce (sect. III) hyperalgesia and allodynia in animals. Induction and expression of hyperalgesia and allodynia are context sensitive. This is discussed in section IV. Neuronal and nonneuronal cell populations have been identified that are indispensable for the induction and/or the expression of hyperalgesia and allodynia as summarized in section V. This review focuses on highly topical spinal mechanisms of hyperalgesia and allodynia including intrinsic and synaptic plasticity, the modulation of inhibitory control (sect. VI), and neuroimmune interactions (sect. VII). The scientific use of language improves also in the field of pain research. Refined definitions of some technical terms including the new definitions of hyperalgesia and allodynia by the International Association for the Study of Pain are illustrated and annotated in section I.

Contralateral high or a combination of high- and low-frequency transcutaneous electrical nerve stimulation reduces mechanical allodynia and alters dorsal horn neurotransmitter content in neuropathic rats

The purpose of the study was to examine the effect of 3 different application strategies for transcutaneous electrical nerve stimulation (TENS) on neuropathy-induced allodynia and dorsal horn neurotransmitter content. Rats were treated with high-frequency, low-frequency, or a combination of high and low-frequency stimulation. TENS was delivered through self-adhesive electrodes daily for 1 hour to rats with a right-sided chronic constriction injury (CCI). Stimulation was delivered to skin or acupuncture points on the left and mechanical and thermal pain thresholds were assessed in the right hind paw. Neurotransmitter content was assessed bilaterally in the dorsal horn of the spinal cord. Daily, high-frequency or a combination of high- and low-frequency TENS reduced mechanical (P < .001), but not thermal allodynia in the right hind paw when compared with untreated CCI rats. Daily high frequency TENS elevated the dorsal horn synaptosomal content of GABA bilaterally (P < .014) and a combination of high- and low-frequency TENS elevated the dorsal horn content of aspartate (P < .001), glutamate (P < .001) and glycine (P < .001) bilaterally over that seen in untreated CCI rats. The present findings support a contralateral approach to the application of TENS and suggest that distinct strategies for TENS application may differentially alter neurotransmission in the central nervous system.

PERSPECTIVE

Because CCI rats are reminiscent of humans with neuropathy, daily high or a combination of high- and low-frequency TENS may reduce mechanical allodynia in humans with neuropathic pain. Because the 2 intervention strategies produce distinctive alterations in spinal cord neurotransmitter content, each may represent a distinctive option for treatment.

Nociceptive behavior in animal models for peripheral neuropathy: spinal and supraspinal mechanisms

Since the initial description by Wall [Wall, P.D., 1967. The laminar organization of dorsal horn and effects of descending impulses. J. Neurophysiol. 188, 403-423] of tonic descending inhibitory control of dorsal horn neurons, several studies have aimed to characterize the role of various brain centers in the control of nociceptive input to the spinal cord. The role of brainstem centers in pain inhibition has been well documented over the past four decades. Lesion to peripheral nerves results in hypersensitivity to mild tactile or cold stimuli (allodynia) and exaggerated response to nociceptive stimuli (hyperalgesia), both considered as cardinal signs of neuropathic pain. The increased interest in animal models for peripheral neuropathy has raised several questions concerning the rostral conduction of the neuropathic manifestations and the role of supraspinal centers, especially brainstem, in the inhibitory control or in the abnormal contribution to the maintenance and facilitation of neuropathic-like behavior. This review aims to summarize the data on the ascending and descending modulation of neuropathic manifestations and discusses the recent experimental data on the role of supraspinal centers in the control of neuropathic pain. In particular, the review emphasizes the importance of the reciprocal interconnections between the analgesic areas of the brainstem and the pain-related areas of the forebrain. The latter includes the cerebral limbic areas, the prefrontal cortex, the intralaminar thalamus and the hypothalamus and play a critical role in the control of pain considered as part of an integrated behavior related to emotions and various homeostatic regulations. We finally speculate that neuropathic pain, like extrapyramidal motor syndromes, reflects a disorder in the processing of somatosensory information.

Noradrenergic pain modulation

Norepinephrine is involved in intrinsic control of pain. Main sources of norepinephrine are sympathetic nerves peripherally and noradrenergic brainstem nuclei A1-A7 centrally. Peripheral norepinephrine has little influence on pain in healthy tissues, whereas in injured tissues it has variable effects, including aggravation of pain. Its peripheral pronociceptive effect has been associated with injury-induced expression of novel noradrenergic receptors, sprouting of sympathetic nerve fibers, and pronociceptive changes in the ionic channel properties of primary afferent nociceptors, while an interaction with the immune system may contribute in part to peripheral antinociception induced by norepinephrine. In the spinal cord, norepinephrine released from descending pathways suppresses pain by inhibitory action on alpha-2A-adrenoceptors on central terminals of primary afferent nociceptors (presynaptic inhibition), by direct alpha-2-adrenergic action on pain-relay neurons (postsynaptic inhibition), and by alpha-1-adrenoceptor-mediated activation of inhibitory interneurons. Additionally, alpha-2C-adrenoceptors on axon terminals of excitatory interneurons of the spinal dorsal horn possibly contribute to spinal control of pain. At supraspinal levels, the pain modulatory effect by norepinephrine and noradrenergic receptors has varied depending on many factors such as the supraspinal site, the type of the adrenoceptor, the duration of the pain and pathophysiological condition. While in baseline conditions the noradrenergic system may have little effect, sustained pain induces noradrenergic feedback inhibition of pain. Noradrenergic systems may also contribute to top-down control of pain, such as induced by a change in the behavioral state. Following injury or inflammation, the central as well as peripheral noradrenergic system is subject to various plastic changes that influence its antinociceptive efficacy.

Effects of electroacupuncture on cold allodynia in a rat model of neuropathic pain: mediation by spinal adrenergic and serotonergic receptors

The present study was performed to examine the effects of electroacupuncture (EA) on cold allodynia and its mechanisms related to the spinal adrenergic and serotonergic systems in a rat model of neuropathic pain. For the neuropathic surgery, the right superior caudal trunk was resected at the level between S1 and S2 spinal nerves innervating the tail. Two weeks after the nerve injury, EA stimulation (2 or 100 Hz) was delivered to Zusanli (ST36) for 30 min. The behavioral signs of cold allodynia were evaluated by the tail immersion test [i.e., immersing the tail in cold water (4 degrees C) and measuring the latency to an abrupt tail movement] before and after the stimulation. And then, we examined the effects of intrathecal injection of prazosin (alpha1-adrenoceptor antagonist, 30 microg), yohimbine (alpha2-adrenoceptor antagonist, 30 microg), NAN-190 (5-HT1A antagonist, 15 microg), ketanserin (5-HT2A antagonist, 30 microg), and MDL-72222 (5-HT3 antagonist, 12 microg) on the action of EA stimulation. Although both 2 Hz and 100 Hz EA significantly relieved the cold allodynia signs, 2 Hz EA induced more robust effects than 100 Hz EA. In addition, intrathecal injection of yohimbine, NAN-190, and MDL-72222, but not prazosin and ketanserin, significantly blocked the relieving effects of 2 Hz EA on cold allodynia. These results suggest that low-frequency (2 Hz) EA is more suitable for the treatment of cold allodynia than high-frequency (100 Hz) EA, and spinal alpha2-adrenergic, 5-HT1A and 5-HT3, but not alpha1-adrenergic and 5-HT2A, receptors play important roles in mediating the relieving effects of 2 Hz EA on cold allodynia in neuropathic rats.

Spinal noradrenaline transporter inhibition by reboxetine and Xen2174 reduces tactile hypersensitivity after surgery in rats

Spinal noradrenaline (NA) released in response to noxious stimuli may play an important role in suppression of nociceptive transmission. Here, we investigated the efficacy of a competitive NA transporter inhibitor (reboxetine) and a noncompetitive NA transporter inhibitor peptide, Xen2174, isolated from the Pacific cone snail, to treat tactile hypersensitivity following paw incisional surgery. Male Sprague-Dawley rats were anesthetized, an incision of the plantar aspect of the hind paw was performed, and withdrawal threshold to von Frey filaments near the surgical site determined. Reboxetine (0.5-5 microg) and Xen2174 (0.3-100 microg) increased withdrawal threshold when injected 24h after paw incision, with a peak effect at 15-60 min, for Xen2174, an ED50 value of 0.64 microg. Administration of Xen2174 (3-30 microg) 15 min before incision also reduced hypersensitivity in a dose-dependent manner. Withdrawal threshold after the single 30 microg dose was greater than vehicle control even at 2, 3, and 5 days after incision. Doses <or=30 microg did not alter spontaneous behavior. The anti-hypersensitivity effect of 10 microg of Xen2174 was totally blocked by the alpha2-adrenoceptor antagonist, idazoxan, and partially blocked by the muscarinic antagonist, atropine. These data suggest that selective NA transporter inhibition suppresses post-incisional hypersensitivity through a different mechanism from that of neuropathic pain, since we previously reported that reversal of hypersensitivity by intrathecal clonidine, an alpha2-adrenoceptor agonist, following spinal nerve ligation is completely blocked by intrathecal atropine. Finally, these data suggest that intrathecal administration of Xen2174 at the time of spinal anesthesia might produce postoperative analgesia in humans.

Administration of MK-801 decreases c-Fos expression in the trigeminal sensory nuclear complex but increases it in the midbrain during experimental movement of rat molars

Various studies reported c-Fos expression in the neurons in the trigeminal sensory nuclear complex (TSNC) following experimental tooth movement, which implies pain transmission to the central nervous system. Meanwhile, MK-801, a noncompetitive antagonist of N-methyl-D-aspartate (NMDA) receptors, was shown to markedly reduce the expression of c-Fos in the trigeminal subnucleus caudalis (Vc) following noxious stimulation but to enhance c-Fos expression markedly in other brain regions, i.e., the neocortex, dorsal raphe and thalamic nuclei. In the present study, we examined the nature of c-Fos expression in the brainstem including the TSNC and midbrain following administration of MK-801 and/or experimental movement of the rat molars. Twelve hours after the beginning of experimental tooth movement, c-Fos was expressed bilaterally in the superficial laminae of Vc (Vc I/II), dorsomedial areas of the trigeminal subnucleus oralis (Vodm) and rostro-dorsomedial areas of the trigeminal subnucleus oralis (Vor) with the ipsilaterally dominant distribution, but hardly in the periaqueductal gray (PAG), dorsal raphe nucleus (DR) and Edinger-Westphal nucleus (EW). Intraperitoneal administration of MK-801 (0.03, 0.3 and 3.0 mg/kg) prior to the onset of experimental tooth movement reduced c-Fos in the TSNC (Vc I/II, Vodm and Vor) but increased it in the nucleus raphe magnus (NRM), ventrolateral PAG (vl PAG), DR and EW. These results highly emphasize that during experimental tooth movement, a blockade of NMDA receptors induces neuronal suppression in the TSNC but increases neuronal activity in the descending antinociceptive system including the NRM, vl PAG, DR and EW.

Nociceptive response to innocuous mechanical stimulation is mediated via myelinated afferents and NK-1 receptor activation in a rat model of neuropathic pain

Peripheral nerve injury in humans can produce a persistent pain state characterized by spontaneous pain and painful responses to normally innocuous stimuli (allodynia). Here we attempt to identify some of the neurophysiological and neurochemical mechanisms underlying neuropathic pain using an animal model of peripheral neuropathy induced in male Sprague-Dawley rats by placing a 2-mm polyethylene cuff around the left sciatic nerve according to the method of Mosconi and Kruger. von Frey hair testing confirmed tactile allodynia in all cuff-implanted rats before electrophysiological testing. Rats were anesthetized and spinalized for extracellular recording from single spinal wide dynamic range neurons (L(3-4)). In neuropathic rats (days 11-14 and 42-52 after cuff implantation), ongoing discharge was greater and hind paw receptive field size was expanded compared to control rats. Activation of low-threshold sensory afferents by innocuous mechanical stimulation (0.2 N for 3 s) in the hind paw receptive field evoked the typical brief excitation in control rats. However, in neuropathic rats, innocuous stimulation also induced a nociceptive-like afterdischarge that persisted 2-3 min. This afterdischarge was never observed in control rats, and, in this model, is the distinguishing feature of the spinal neural correlate of tactile allodynia. Electrical stimulation of the sciatic nerve at 4 and at 20 Hz each produced an initial discharge that was identical in control and in neuropathic rats. This stimulation also produced an afterdischarge that was similar at the two frequencies in control rats. However, in neuropathic rats, the afterdischarge produced by 20-Hz stimulation was greater than that produced by 4-Hz stimulation. Given that acutely spinalized rats were studied, only peripheral and/or spinal mechanisms can account for the data obtained; as synaptic responses from C fibers begin to fail above approximately 5-Hz stimulation [Pain 46 (1991) 327], the afterdischarge in response to 20-Hz stimulation suggests a change mainly in myelinated afferents and a predominant role of these fibers in eliciting this afterdischarge. These data are consistent with the suggestion that peripheral neuropathy induces phenotypic changes predominantly in myelinated afferents, the sensory neurons that normally respond to mechanical stimulation. The NK-1 receptor antagonist, CP-99,994 (0.5 mg/kg, i.v.), depressed the innocuous pressure-evoked afterdischarge but not the brief initial discharge of wide dynamic range neurons, and decreased the elevated ongoing rate of discharge in neuropathic rats. These results support the concept that following peripheral neuropathy, myelinated afferents may now synthesize and release substance P. A result of this is that tonic release of substance P from the central terminals of these phenotypically altered neurons would lead to ongoing excitation of NK-1-expressing nociceptive spinal neurons. In addition, these spinal neurons would also exhibit exaggerated responses to innocuous pressure stimulation. The data in this study put forth a possible neurophysiological and neurochemical basis of neuropathic pain and identify substance P and the NK-1 receptor as potential neurochemical targets for its management.

Joint manipulation reduces hyperalgesia by activation of monoamine receptors but not opioid or GABA receptors in the spinal cord

Joint manipulation has long been used for pain relief. However, the underlying mechanisms for manipulation-related pain relief remain largely unexplored. The purpose of the current study was to determine which spinal neurotransmitter receptors mediate manipulation-induced antihyperalgesia. Rats were injected with capsaicin (50 microl, 0.2%) into one ankle joint and mechanical withdrawal threshold measured before and after injection. The mechanical withdrawal threshold decreases 2 h after capsaicin injection. Two hours after capsaicin injection, the following drugs were administered intrathecally: bicuculline, blocks gamma-aminobutyric acid (GABAA) receptors; naloxone, blocks opioid receptors; yohimbine blocks, alpha2-adrenergic receptors; and methysergide, blocks 5-HT(1/2) receptors. In addition, NAN-190, ketanserin, and MDL-72222 were administered to selectively block 5-HT1A, 5-HT2A, and 5-HT3 receptors, respectively. Knee joint manipulation was performed 15 min after administration of drug. The knee joint was flexed and extended to end range of extension while the tibia was simultaneously translated in an anterior to posterior direction. The treatment group received three applications of manipulation, each 3 min in duration separated by 1 min of rest. Knee joint manipulation after capsaicin injection into the ankle joint significantly increases the mechanical withdrawal threshold for 45 min after treatment. Spinal blockade of 5-HT(1/2) receptors with methysergide prevented, while blockade of alpha2-adrenergic receptors attenuated, the manipulation-induced antihyperalgesia. NAN-190 also blocked manipulation-induced antihyperalgesia suggesting that effects of methysergide are mediated by 5-HT1A receptor blockade. However, spinal blockade of opioid or GABAA receptors had no effect on manipulation induced-antihyperalgesia. Thus, the antihyperalgesia produced by joint manipulation appears to involve descending inhibitory mechanisms that utilize serotonin and noradrenaline.

Selective loss of spinal GABAergic or glycinergic neurons is not necessary for development of thermal hyperalgesia in the chronic constriction injury model of neuropathic pain

GABA and glycine are inhibitory neurotransmitters used by many neurons in the spinal dorsal horn, and intrathecal administration of GABA(A) and glycine receptor antagonists produces behavioural signs of allodynia, suggesting that these transmitters have an important role in spinal pain mechanisms. Several studies have described a substantial loss of GABA-immunoreactive neurons from the dorsal horn in nerve injury models, and it has been suggested that this may be associated with a loss of inhibition, which contributes to the behavioural signs of neuropathic pain. We have carried out a quantitative stereological analysis of the proportions of neurons in laminae I, II and III of the rat dorsal horn that show GABA- and/or glycine-immunoreactivity 2 weeks after nerve ligation in the chronic constriction injury (CCI) model, as well as in sham-operated and nai;ve animals. At this time, rats that had undergone CCI showed a significant reduction in the latency of withdrawal of the ipsilateral hindpaw to a radiant heat stimulus, suggesting that thermal hyperalgesia had developed. However, we did not observe any change in the proportion of neurons in laminae I-III of the ipsilateral dorsal horn that showed GABA- or glycine-immunoreactivity compared to the contralateral side in these animals, and these proportions did not differ significantly from those seen in sham-operated or nai;ve animals. In addition, we did not see any evidence for alterations of GABA- or glycine-immunostaining in the neuropil of laminae I-III in the animals that had undergone CCI. Our results suggest that significant loss of GABAergic or glycinergic neurons is not necessary for the development of thermal hyperalgesia in the CCI model of neuropathic pain.

Superficial NK1-expressing neurons control spinal excitability through activation of descending pathways

The increase in pain sensitivity that follows injury is regulated by superficially located projection neurons in the dorsal horn of the spinal cord that express the neurokinin-1 (NK1) receptor. After selective ablation of these neurons in rats, we identified changes in receptive field size, mechanical and thermal coding and central sensitization of deeper dorsal horn neurons that are important for both pain sensations and reflexes. We were able to reproduce these changes by pharmacological block of descending serotonergic facilitatory pathways. Using Fos histochemistry, we found changes in the activation of serotonergic neurons in the brainstem as well as evidence for a loss of descending control of spinal excitability. We conclude that NK1-positive spinal projection neurons, activated by primary afferent input, project to higher brain areas that control spinal excitability--and therefore pain sensitivity--primarily through descending pathways from the brainstem.

Electrophysiologic characterization of the antinociceptive actions of S18616, a novel and potent alpha2-adrenoceptor agonist, after acute and persistent pain states

alpha (2)-Adrenoceptor (AR) agonists are active in behavioral models of persistent pain involving tissue and nerve damage. We evaluated the spinal effect of a novel, potent, and selective alpha (2)-AR agonist, [7,8](2-chlorobenzo)-2-amino-1-aza-3-oxa[4,5]spirodeca-1,7-diene (S18616), on the responses of dorsal horn neurons in halothane-anesthetized rats. Intrathecal administration of S18616 (0.1 to 3.0 microg) dose-dependently suppressed C- and A delta-fiber evoked responses but not the A beta-fiber evoked response. Drug effects were reversed by the alpha (2)-AR antagonists, atipamezole and idazoxan (100 microg). In rats with unilateral spinal nerve (L5-L6) ligation performed 2 weeks before study, S18616 (0.1 to 3.0 microg) dose-dependently suppressed the C- and A delta-fiber evoked responses and blocked "wind-up" in these neurons. The potency was comparable between nerve-injured and sham-operated rats, and S18616 was equally effective against responses to thermal and high-intensity mechanical stimuli. Interestingly, the effectiveness of S18616 on the low-intensity mechanical evoked response was significantly enhanced after nerve injury. Finally, S18616 (0.3 and 3.0 microg) reduced the neuronal responses produced by intraplantar injection of formalin. In conclusion, S18616 dose-dependently and potently inhibits the responses of dorsal horn neurons to peripheral stimulation in normal, inflamed, and neuropathic rats. These data support the use of spinal S18616 and other alpha (2)-AR agonists in the management of clinical pain.

Dorsal horn synaptosomal content of aspartate, glutamate, glycine and GABA are differentially altered following chronic constriction injury to the rat sciatic nerve

The present study directly examined the axon terminal (synaptosomal) level of amino acid neurotransmitters in rats following chronic constriction injury (CCI) to the sciatic nerve. Dorsal horn, synaptosomal content of aspartate (Asp), glutamate (Glu), glycine and gamma-aminobutyric acid (GABA) was assessed in these rats, and in normal age-matched and younger rats. The synaptosomal content of Asp and Glu in CCI rats was increased by 44-46% compared with control rats (P<0.016). The synaptosomal content of GABA in younger rats was 33% lower than that observed in control rats (P<0.005). Altered axon terminal levels of amino acid neurotransmitters accompany the painful symptoms of neuropathy. The lower axon terminal level of GABA in younger rats may help to explain the age-dependency of pain development in animal models of nerve injury.

Descending control of pain

Upon receipt in the dorsal horn (DH) of the spinal cord, nociceptive (pain-signalling) information from the viscera, skin and other organs is subject to extensive processing by a diversity of mechanisms, certain of which enhance, and certain of which inhibit, its transfer to higher centres. In this regard, a network of descending pathways projecting from cerebral structures to the DH plays a complex and crucial role. Specific centrifugal pathways either suppress (descending inhibition) or potentiate (descending facilitation) passage of nociceptive messages to the brain. Engagement of descending inhibition by the opioid analgesic, morphine, fulfils an important role in its pain-relieving properties, while induction of analgesia by the adrenergic agonist, clonidine, reflects actions at alpha(2)-adrenoceptors (alpha(2)-ARs) in the DH normally recruited by descending pathways. However, opioids and adrenergic agents exploit but a tiny fraction of the vast panoply of mechanisms now known to be involved in the induction and/or expression of descending controls. For example, no drug interfering with descending facilitation is currently available for clinical use. The present review focuses on: (1) the organisation of descending pathways and their pathophysiological significance; (2) the role of individual transmitters and specific receptor types in the modulation and expression of mechanisms of descending inhibition and facilitation and (3) the advantages and limitations of established and innovative analgesic strategies which act by manipulation of descending controls. Knowledge of descending pathways has increased exponentially in recent years, so this is an opportune moment to survey their operation and therapeutic relevance to the improved management of pain.

Modulatory actions of serotonin, norepinephrine, dopamine, and acetylcholine in spinal cord deep dorsal horn neurons

The deep dorsal horn represents a major site for the integration of spinal sensory information. The bulbospinal monoamine transmitters, released from serotonergic, noradrenergic, and dopaminergic systems, exert modulatory control over spinal sensory systems as does acetylcholine, an intrinsic spinal cord biogenic amine transmitter. Whole cell recordings of deep dorsal horn neurons in the rat spinal cord slice preparation were used to compare the cellular actions of serotonin, norepinephrine, dopamine, and acetylcholine on dorsal root stimulation-evoked afferent input and membrane cellular properties. In the majority of neurons, evoked excitatory postsynaptic potentials were depressed by the bulbospinal transmitters serotonin, norepinephrine, and dopamine. Although, the three descending transmitters could evoke common actions, in some neurons, individual transmitters evoked opposing actions. In comparison, acetylcholine generally facilitated the evoked responses, particularly the late, presumably N-methyl-D-aspartate receptor-mediated component. None of the transmitters modified neuronal passive membrane properties. In contrast, in response to depolarizing current steps, the biogenic amines significantly increased the number of spikes in 14/19 neurons that originally fired phasically (P < 0.01). Together, these results demonstrate that even though the deep dorsal horn contains many functionally distinct subpopulations of neurons, the bulbospinal monoamine transmitters can act at both synaptic and cellular sites to alter neuronal sensory integrative properties in a rather predictable manner, and clearly distinct from the actions of acetylcholine.

Differential effects of NMDA and group I mGluR antagonists on both nociception and spinal cord protein kinase C translocation in the formalin test and a model of neuropathic pain in rats

Coincident with nociception, both noxious chemical stimulation of the hind paw and chronic constriction injury (CCI) of the sciatic nerve produce an increase in protein kinase C (PKC) translocation in the spinal cord of rats. Noxious stimulus-induced PKC translocation likely depends on glutamate activity at either N-methyl-D-aspartate (NMDA) receptors or group I metabotropic glutamate receptors (mGluR1/5) in the spinal cord dorsal horn. This study compares nociceptive responses to, and the alterations in membrane-associated PKC, induced by noxious chemical stimulation of the hindpaw and CCI of the sciatic nerve, as well as their modulation by both NMDA and mGluR1/5 receptor antagonists. Three groups of rats were given a single intrathecal (i.t.) injection of either vehicle, dizocilpine maleate (MK-801, 60 nmol), an NMDA receptor antagonist, or (S)-4-carboxyphenylglycine (S)-4CPG, (150 nmol), an mGluR1/5 antagonist, 10 min prior to a 50 microl of 2.5% formalin injection into the ventral surface of one hind paw. Another three groups of rats were given twice daily injections of either vehicle, MK-801 (30 nmol) or (S)-4CPG (90 nmol) i.t. for 5 days starting 30 min before CCI or sham injury of the sciatic nerve. Nociceptive responses were assessed for a 60 min period after the formalin injection in the first three groups, and tests of mechanical and cold allodynia were performed on days 4, 8, 12 and 16 after CCI for the latter three groups. Furthermore, changes in the levels of membrane-associated PKC, as assayed by quantitative autoradiography of the specific binding of [3H]-phorbol 12,13-dibutyrate ([3H]-PDBu) in the dorsal horn of the lumbar spinal cord sections, were assessed in formalin-injected rats (at 5, 25 and 60 min) and in neuropathic rats 5 days after CCI, treated (as above) with vehicle, MK-801 or (S)-4CPG. The results indicate that i.t. treatment with MK-801 significantly reduced nociceptive scores in the formalin test and also produced a significant suppression of formalin-induced increases in [3H]-PDBu binding in laminae I-II, III-VI and X of the lumbar spinal cord. In contrast, i.t. treatment with (S)-4CPG failed to significantly affect either nociceptive behaviours in the formalin test or formalin-induced increases in [3H]-PDBu binding in laminae I-II and III-VI of the lumbar spinal cord. On the other hand, i.t. treatment with either MK-801 or (S)-4CPG produced a significant reduction in mechanical and cold hypersensitivity, as well as [3H]-PDBu binding in laminae I-II and III-VI of the lumbar spinal cord, after CCI. These results suggest that while NMDA, but not mGluR1/5, receptors are involved in translocation of PKC and nociception in a model of persistent acute pain, both types of receptors influence the translocation of PKC in dorsal horn and mechanical and cold allodynia in a model of chronic neuropathic pain.

Antiallodynic effect of intrathecally administered 5-HT(2) agonists in rats with nerve ligation

We examined the antiallodynic effect of intrathecally administered serotonin receptor agonists including 5-HT(1A), 5-HT(1B), 5-HT(2) and 5-HT(3) receptor subtypes in a rat model using spinal nerve ligation at L5 and L6. Administration of the 5-HT(2) receptor agonist, alpha-methyl-5-hydroxytryptamine maleate (alpha-m-5-HT; 3-100 microg) or (+/-)-1-(4-iodo-2,5-dimethoxyphenyl)-2-aminopropane hydrochloride (DOI; 10-100 microg), showed dose-dependent antiallodynic actions with no associated motor weakness. The antiallodynic action of alpha-m-5-HT was more potent than that of DOI. The effects of 5-HT(2) agonists on tactile allodynia were reversed by intrathecal pretreatment with the selective 5-HT(2) antagonist ketanserin and with the mixed 5-HT(1) and 5-HT(2) antagonist methysergide. Neither the mixed 5-HT(1A) and 5-HT(1B) antagonist cyanopindolol nor the selective 5-HT(3) antagonist MDL72222 attenuated antiallodynic effects induced by 5-HT(2) agonists. In contrast, the selective 5-HT(1A) agonist 8-hydroxy-2-(di-n-propylamino)-tetralin hydrobromide (8-OH-DPAT; 1-50 microg), the 5-HT(1B) agonist 5-methoxy-3-(1,2,5,6-tetrahydro-4-pyridinil)-1H-indol (RU-24969; 10-100 microg) and the 5-HT(3) agonist 2-methyl-5-hydroxytryptamine maleate (2-m-5-HT; 30-300 microg) all lacked significant antiallodynic action with intrathecal administration. These results indicate that the 5-HT(2) receptor plays an essential role in spinal suppression of neuropathic pain by 5-HT.

Time-dependent changes in neurokinin(3) receptors and tachykinins during adjuvant-induced peripheral inflammation in the rat

Although considerable evidence exists that spinal neurokinin(1) receptors are involved in central sensitization of nociception, recent evidence from knockout studies indicates that other neurokinin receptors in the spinal cord may mediate a portion of the hyperalgesia caused by substance P and neurokinin A. The present study determined whether the second most abundant class of neurokinin receptors, neurokinin(3) receptors, are regulated during persistent peripheral inflammation. Inflammation in the hind paw of the rat was induced by intraplantar injection of complete Freund's adjuvant. Receptor autoradiography revealed specific binding of [125I]-MePhe(7)-NKB, a selective ligand for neurokinin(3) receptors, in the superficial dorsal horn of the spinal cord. Specific binding of [125I]-MePhe(7)-NKB in the medial dorsal horn was reduced bilaterally two days after unilateral injection of complete Freund's adjuvant. Binding returned to basal levels four days after injection of complete Freund's adjuvant. Neurokinin(3) receptor messenger RNA levels doubled in the dorsal spinal cord at 12h and remained elevated for at least four days. The change in neurokinin(3) receptor binding and messenger RNA during adjuvant-induced inflammation may be a consequence of activation of the receptor. Spinal levels of potential endogenous ligands for spinal neurokinin(3) receptors were measured by radioimmunohistochemistry. Immunoreactive substance P but not neurokinin B peptide 2, a marker for neurokinin B, was reduced bilaterally during adjuvant-induced inflammation.Collectively, these data indicate that spinal neurokinin(3) receptors may play a role in spinal neurotransmission of injured rats and require consideration of other tachykinins as physiologically relevant ligands to spinal neurokinin(3) receptors.

Medullary monoamine levels during experimental tooth movement

We investigated possible influence of nociception induced by experimental tooth movement on the medullary monoaminergic inhibitory systems. Forty-eight hours after the start of the experimental tooth movement, significant increases in dorsal serotonin (5-HT), dopamine (DA), norepinephrine (NE), 5-hydroxyindoleacetic acid (5-HIAA), and dihydroxyphenylacetic acid (DOPAC) levels were detected with ipsilateral dominance. These results suggest that the nociception induced by experimental tooth movement might be under modulation of serotonergic, noradrenergic, and dopaminergic systems.

Changes in spinal serotonin turnover mediate age-related differences in the behavioral manifestations of peripheral nerve injury

The Bennett and Xie model of peripheral nerve injury was used to study the effects of aging on the onset and progression of sciatic nerve ligation (SNL)-induced thermal hyperalgesia and tactile-evoked allodynia in young, mature, and aged Fischer 344 FBNF1 male rats (4-6, 14-16, and 24-26 months old, respectively). A plantar analgesia meter and calibrated von Frey pressure filaments were employed as the analgesiometric assays. In the absence of nerve injury, aged rats were found to be more sensitive than younger animals to noxious thermal stimuli. Following the SNL surgery, thermal hyperalgesia was observed in all three age groups within 3 days. On post-SNL day 35, the paw-withdrawal latency values of the young and mature animals returned to presurgical baseline levels, while the aged rats continued to exhibit thermal hyperalgesia. Tactile-evoked allodynia was apparent within 3 days following peripheral nerve injury in the oldest cohort, but was delayed in the younger animals. On post-SNL days 0 (control), 3, 21, and 35, young, mature, and aged rats were sacrificed and high-performance liquid chromatography and electrochemical detection (HPLC/ECD) methods were used for neurochemical analyses of spinal serotonin (5-HT), norepinephrine (NE), and 5-hydroxyindoleacetic acid (5-HIAA). Spinal 5-HT and NE levels were not significantly altered by the aging process, nor were they affected by peripheral nerve injury. However, spinal 5-HT turnover from the aged animals was greater than that detected in spinal tissue from the younger counterparts. Differences in spinal 5-HT turnover may contribute to age-related variability in spinal nociceptive processing.

Effects of midazolam in the spinal nerve ligation model of neuropathic pain in rats

Potential changes in the spinal GABAergic activity after nerve injury were studied by comparing the effects of systemic administration of the benzodiazepine midazolam on the noxious evoked responses of dorsal horn in rats with spinal nerve ligation of neuropathy and control animals. The tight ligation of the L(5) and L6 spinal nerves was performed in adult male Sprague-Dawley rats and resulting mechanical and cold allodynia were assessed with von Frey hairs and the acetone drop test. Single unit extracellular recordings of dorsal horn neurones were performed 15-18 days after the surgery under halothane anaesthesia using transcutaneous electrical stimulation of the receptive field at three times the C-fibre threshold. The rats in the spinal nerve ligation group, but not in the sham-operated control group developed mechanical and cold allodynia. Subcutaneous administration of midazolam 0.1-3.0 mg/kg reduced the Adelta-fibre evoked activity in a dose-related manner in all study groups, but the C-fibre evoked activity was significantly reduced only in the spinal nerve ligation group. The inhibitory effects of s.c. midazolam were significantly reversed by i.t. administration of flumazenil, suggesting a spinal site of action. Midazolam reduced C-fibre evoked firing significantly more in the spinal nerve ligation model than in the non-operated or sham controls. These results indicate changes in the spinal GABAergic system in the neuropathic animals and could be of importance in the development of new treatments for neuropathic pain.

Metabolic activity changes in the rat spinal cord during adjuvant monoarthritis

The development of chronic pain is associated with activity-dependent plastic changes in neuronal structures in the peripheral and central nervous system. In order to investigate the time-dependent processing of afferent noxious stimuli in the spinal cord we employed the quantitative autoradiographic 2-deoxyglucose technique in a model of chronic monoarthritic pain in the rat. Spinal metabolic activity was determined at various time-points (two, four and 14 days) after the injection of complete Freund's adjuvant into the left tibiotarsal joint. In addition, the effect of acute noxious mechanical stimulation of the arthritic joint was investigated at 14 days of monoarthritis. Local glucose utilization was determined in lumbar segments L2-L5, ipsi- and contralateral to the inflamed hindpaw, and compared with saline-injected controls. In general, monoarthritic animals had bilaterally increased metabolic activity in all laminae of the spinal cord. Detailing the time-course showed that in rats with two days of monoarthritis metabolic activity was significantly increased to a similar extent on both sides of all spinal laminae. In contrast, at four days, glucose utilization in deep laminae of the dorsal horn (laminae V-VI), the central gray area (laminae X) and the ventral horn (laminae VII-IX) tended to return to control levels. At 14 days of monoarthritis, however, metabolic activity showed a further increase in all laminae of the spinal cord. This increase was more pronounced on the side ipsilateral to inflammation, reaching 65% above corresponding control levels in laminae V, VI. Animals with 14 days of monoarthritis which were subjected to mechanical noxious stimulation of the arthritic joint displayed clear behavioral signs of acute pain. Although in this group metabolic activity was above control levels, it was lower than in animals with 14 days of monoarthritis that were not additionally stimulated. The data show not only a general increase of spinal cord metabolic activity during the time-course of the development of a chronic pain state, but also show a region-specific non-linear time profile. This may reflect the complexity of transducing and suppressive transmitter systems involved in the central processing of ongoing pain.

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.