Descending modulation of Fos expression after persistent peripheral inflammation

Propriospinal pathways in the dorsal horn (laminae I-IV) of the rat lumbar spinal cord

The spinal dorsal horn is regarded as a unit that executes the function of sensory information processing without any significant communication with other regions of the spinal gray matter. Within the spinal dorsal horn, however, the different rostro-caudal and medio-lateral subdivisions intensively communicate with each other through propriospinal pathways. This review gives an overview about these propriospinal systems, and emphasizes that the medial and lateral parts of the spinal dorsal horn show the following distinct features in their propriospinal interconnectivities: (a) A 100-300μm long section of the medial aspects of laminae I-IV projects to and receives afferent fibers from a three segment long compartment of the spinal dorsal gray matter, whereas the same length of the lateral aspects of laminae I-IV projects to and receives afferent fibers from the entire rostro-caudal extent of the lumbar spinal cord. (b) The medial aspects of laminae I-IV project extensively to the lateral areas of the dorsal horn. In contrast to this, the lateral areas of laminae I-IV, with the exception of a few fibers at the segmental level, do not project back to the medial territories. (c) There is a substantial direct commissural connection between the lateral aspects of laminae I-IV on the two sides of the lumbar spinal cord. The medial part of laminae I-IV, however, establishes only a minor commissural propriospinal connection with the gray matter on the opposite side.

Long-term changes of c-Fos expression in the rat spinal cord following chronic constriction injury

The expression of c-Fos protein has been used as a relative marker of nociceptive neuronal activity in the spinal cord following various noxious stimuli. Experiments were conducted to examine c-Fos expression in lumbar spinal cord (L3-L6) following chronic constriction injury (CCI) in relation to nociceptive behavior over longer survival period up to 28 days. Development of mechanical allodynia was observed in the ipsilateral hind paw of CCI rats at day 3 and lasted up to 28 days. In contrast, the spinal c-Fos expression in CCI rats appeared in a biphasic manner. The highest number of c-Fos positive neurons occurred during the first week, followed by a decline at 7 and 14 days and reappearance at day 28 following injury. The early increase of c-Fos expression correlated with allodynia development, however, at longer survival period (28 days) c-Fos positivity become comparable in both CCI and sham groups despite their obvious behavior differences. Our results suggest that, at least in the CCI model, the c-Fos protein expression should not be considered as a reliable index of pain sensation disorders.

Discovery of novel selective norepinephrine inhibitors: 1-(2-morpholin-2-ylethyl)-3-aryl-1,3-dihydro-2,1,3-benzothiadiazole 2,2-dioxides (WYE-114152)

Sequential modification of the previously identified 4-[3-aryl-2,2-dioxido-2,1,3-benzothiadiazol-1(3H)-yl]-1-(methylamino)butan-2-ols led to the identification of a new series of 1-(2-morpholin-2-ylethyl)-3-aryl-1,3-dihydro-2,1,3-benzothiadiazole 2,2-dioxides that are potent and selective inhibitors of the norepinephrine transporter over both the serotonin and dopamine transporters. One representative compound 10b (WYE-114152) had low nanomolar hNET potency (IC(50) = 15 nM) and good selectivity for hNET over hSERT (>430-fold) and hDAT (>548-fold). 10b was additionally bioavailable following oral dosing and demonstrated efficacy in rat models of acute, inflammatory, and neuropathic pain.

Injury induced activation of extracellular signal-regulated kinase (ERK) in the rat rostral ventromedial medulla (RVM) is age dependant and requires the lamina I projection pathway

Descending controls originating in part from the rostral ventromedial medulla (RVM) regulate the excitability of dorsal horn neurons and maintain peripheral pain states. Activation of extracellular signal regulated kinase (ERK) in RVM neurons has been shown following peripheral inflammation and is involved in generating the accompanying inflammatory hyperalgesia. Here, we show that spared nerve injury (SNI), a model of neuropathic pain, results in an increase in ERK activity in RVM neurons of adult rats 3 and 8 days following surgery. We carried out two experimental procedures to demonstrate that this increase in ERK activation was related to the increased mechanical sensitivity associated with SNI. First, we showed that lesions of the lamina I/III ascending pathway from the dorsal horn attenuated both mechanical hyperalgesia and ERK activation in the RVM. Second, we performed SNI in P10 rats. At this age, SNI did not result in mechanical hypersensitivity, as previously shown, and did not activate ERK in the RVM. Finally, the percentage of pERK expressing neurones that were also serotonergic was always around 60%, independent of pain state and age, indicating an important role for serotonin in descending controls of pain states.

1-(Indolin-1-yl)-1-phenyl-3-propan-2-olamines as potent and selective norepinephrine reuptake inhibitors

Efforts to identify new selective and potent norepinephrine reuptake inhibitors (NRIs) for multiple indications by structural modification of the previous 3-(arylamino)-3-phenylpropan-2-olamine scaffold led to the discovery of a novel series of 1-(indolin-1-yl)-1-phenyl-3-propan-2-olamines (9). Investigation of the structure-activity relationships revealed that small alkyl substitution at the C3 position of the indoline ring enhanced selectivity for the norepinephrine transporter (NET) over the serotonin transporter (SERT). Several compounds bearing a 3,3-dimethyl group on the indoline ring, 9k, 9o,p, and 9s,t, exhibited potent inhibition of NET (IC(50) = 2.7-6.5 nM) and excellent selectivity over both serotonin and dopamine transporters. The best example from this series, 9p, a potent and highly selective NRI, displayed oral efficacy in a telemetric rat model of ovariectomized-induced thermoregulatory dysfunction, a mouse p-phenylquinone (PPQ) model of acute visceral pain, and a rat spinal nerve ligation (SNL) model of neuropathic pain.

Anti-hyperalgesic and anti-inflammatory effects of the modified Chinese herbal formula Huo Luo Xiao Ling Dan (HLXL) in rats

Chinese herbal medicine has been used for thousands of years in China and other Asian countries to treat a variety of inflammatory diseases. The classic Chinese herbal formula, Huo Luo Xiao Ling Dan (HLXL) is commonly used in traditional Chinese herbal medicine for the treatment of joint pain and other symptoms of arthritis. The present study is an investigation of the effects of a modified HLXL extract on persistent hyperalgesia and edema in rats with peripheral inflammation. Inflammation was induced by injecting complete Freund's adjuvant (CFA) into one hind paw. Four dosages of the extract were compared to a vehicle control. Each was administered intragastrally (i.g.) daily for seven days beginning one day before CFA. Hyperalgesia was assessed using a paw withdrawal latency (PWL) test and edema was determined by measuring paw thickness at pre-CFA and 2 hours, 24 hours, and 5 days post-CFA. Immunohistochemistry was performed 2 hours post-CFA to determine spinal Fos protein expression. Adverse effects of the extract were monitored by observing the animals closely for unusual behavioral changes. Compared to the control, HLXL at the two lower dosages (0.575 g/kg and 1.15 g/kg) were effective in the later stage (day 5) of inflammatory hyperalgesia and edema, while the two higher dosages (2.3 g/kg and 4.6 g/kg) alleviated early stage hind paw inflammation and hyperalgesia and facilitated recovery from paw edema and hyperalgesia during the late stage. HLXL at 2.30 g/kg significantly suppressed Fos expression in laminae I-II, III-IV and V-VI ipsilaterally and in III-IV contralaterally. No significant signs of toxicity or adverse effects were observed. The data suggest that HLXL dosage-dependently attenuates CFA-induced inflammation and hyperalgesia, at least in part by inhibiting noxious transmission at the dorsal horn of the spinal cord.

1- or 3-(3-Amino-2-hydroxy-1-phenyl propyl)-1,3-dihydro-2H-benzimidazol-2-ones: potent, selective, and orally efficacious norepinephrine reuptake inhibitors

Sequential structural modifications of the aryloxypropanamine template (e.g., atomoxetine, 2) led to a novel series of 1-(3-amino-2-hydroxy-1-phenyl propyl)-1,3-dihydro-2H-benzimidazol-2-ones as selective norepinephrine reuptake inhibitors (NRIs). In general, this series of compounds potently blocked the human norepinephrine transporter (hNET) while exhibiting selectivity at hNET against both the human serotonin (hSERT) and dopamine transporters (hDAT). Numerous compounds (e.g., 19-22) had low nonamolar hNET potency with IC(50) values of 7-10 nM and excellent selectivity (>500 fold) at hNET over hSERT and hDAT. Several compounds, such as 20 and 22, were tested in a telemetric rat model of ovariectomized-induced thermoregulatory dysfunction and were efficacious at oral doses of 3 mg/kg in reducing the tail skin temperature. In addition, compound 20 was also studied in the rat hot plate and spinal nerve ligation (SNL) models of acute and neuropathic pain, respectively, and was orally efficacious at doses of 3-10 mg/kg.

Endogenous kappa-opioid receptor systems inhibit hyperalgesia associated with localized peripheral inflammation

Peripheral inflammation evokes functional and biochemical changes in the periphery and spinal cord which result in central sensitization and hypersensitivity. Inhibitory control systems from the rostral ventromedial medulla (RVM) are also activated. The present study investigates whether endogenous kappa-opioid receptor (KOPr) systems contribute to these neuroadaptations. Inflammation was induced by intraplantar injection of complete Freund's adjuvant (CFA) into one hindpaw. Mechanical and thermal thresholds were determined using the Von Frey and radiant heat tests, respectively. KOPr gene deletion in mice or systemic administration of the long-acting KOPr antagonist, norbinaltorphimine (norBNI) significantly exacerbated mechanical and thermal hypersensitivity of the ipsilateral, inflamed paw. Thermal and mechanical thresholds of the non-inflamed, contralateral hindpaw were unaffected by CFA treatment. However, gene deletion as well as norBNI treatment resulted in mechanical, but not thermal hypersensitivity of the non-inflamed paw. Similar results were obtained when norBNI was administered intrathecally or into the RVM in rats. These data demonstrate a previously unrecognized role of endogenous KOPr systems in inhibiting hyperalgesia during inflammation. Furthermore, they demonstrate that decreased KOPr activity in either the spinal cord or RVM not only enhances mechanical and thermal hyperalgesia of the inflamed limb but also leads to an unmasking of mechanical hyperalgesia at a site remote from inflammation. The differential effects of KOPr antagonism on mechanical versus thermal thresholds for the non-inflamed paw support the notion that distinct neuroanatomical or neurochemical mechanisms modulate the processing of thermal versus mechanical stimuli.

Rostral ventral medulla modulation of the visceromotor reflex evoked by urinary bladder distension in female rats

The present studies examined the involvement of the rostral ventral medulla (RVM) in modulating the visceromotor response (VMR) evoked by urinary bladder distension (UBD) in adult female rats. The VMR was indexed by electromyographic (EMG) responses of the abdominal external oblique muscle to UBD. Experiment 1 showed that the predominant effect of electrical stimulation of the RVM in normal rats was to produce intensity-dependent inhibition of the VMR (54% of sites sampled). Facilitatory, biphasic, or no effects were obtained at the remaining sites. Experiment 2 showed that RVM-induced inhibition of the VMR was significantly attenuated by intraperitoneal (i.p.) administration of naloxone but not saline vehicle. In experiment 3, we examined the effect of lesions of the RVM in rats with inflamed bladders because previous research has shown that an endogenous opioid inhibitory system is engaged by bladder inflammation. Electrolytic lesions of the RVM but not sham lesions of the RVM significantly increased the VMR to graded UBD in rats with augmented VMRs induced by prior inflammation of the bladder. The present data suggest that the RVM can inhibit the VMR to UBD, acting in part via an opioid-inhibitory system, and that bladder inflammation can recruit the RVM to produce a net inhibitory effect on the VMR to UBD.

PERSPECTIVE

Stimulation of the RVM resulted in inhibitory, facilitatory, and biphasic modulation of the visceromotor reflex to urinary bladder distension. Inhibitory effects of stimulation were attenuated by naloxone, and lesions of the RVM enhanced the VMR in rats with inflamed bladders. These data indicate an important role of the RVM in modulating bladder pain.

Inflammation-induced changes in rostral ventromedial medulla mu and kappa opioid receptor mediated antinociception

Acute microinjection of mu-, delta-, or kappa-opioid receptor (MOPr, DOPr, KOPr) agonists into the rostral ventromedial medulla (RVM) produces antinociception. Thermal antinociception produced by MOPr and DOPr agonists is potentiated during inflammation [Hurley RW, Hammond DL. The analgesic effects of supraspinal mu and delta opioid receptor agonists are potentiated during persistent inflammation. J Neurosci 2000;20:1249-59]. Whether this potentiation extends to other stimulus modalities or to KOPr agonists is unknown. To examine these issues, rats received a unilateral intraplantar injection of complete Freund's adjuvant (CFA). Antinociception produced by RVM infusion of the KOPr agonist, U69593, and the MOPr agonist, DAMGO, was tested 4h-2 weeks thereafter. Thermal paw withdrawal latencies (PWLs) were assessed using the Hargreaves method. Mechanical thresholds were determined with the Von Frey and Randall-Selitto method. PWLs of the inflamed paw were reduced 4h-2 weeks after CFA injection. Infusion of either U69593 or DAMGO increased PWLs in CFA treated rats. A bilateral enhancement of the response to both agonists was observed 2 weeks relative to 4h post-CFA injection. Mechanical thresholds of the inflamed paw were decreased for >2 weeks post-CFA injection. Infusion of either agonist elevated thresholds of the inflamed and non-inflamed paws of CFA-treated rats. The magnitude of these effects was greater 2 weeks post-CFA injection for DAMGO and increased progressively for U69593. These data demonstrate that RVM infusion of MOPr or KOPr agonists attenuates CFA-evoked thermal and tactile allodynia and that these effects increase during prolonged inflammation. The augmented response of the non-inflamed paw to agonists suggests that inflammation induces centrally-mediated neuroplastic changes which enhance MOPr- and KOPr-mediated antinociception.

C-fos expression at the spinal dorsal horn of streptozotocin-induced diabetic rats

BACKGROUND

Pain during diabetic neuropathy is associated with peripheral nerve damage but recent evidences suggest the occurrence of central effects. We used the activation of the c-fos protooncogene to study the activity of spinal dorsal horn neurons in streptozotocin (STZ)-induced diabetic rats in the absence of stimulation or in response to innocuous or noxious stimuli.

METHODS

Four weeks after saline or STZ (50 mg/kg) injection, rats were anaesthetized and either not further manipulated or submitted to innocuous (gentle touch), noxious mechanical (pinching) or noxious thermal (radiant heat) stimulation of the hindlimb skin. In each experimental situation, the numbers of Fos-immunoreactive (Fos-IR) neurons occurring in the superficial (laminae I-II) or deep (laminae III-V) dorsal horn were compared.

RESULTS

In the absence of stimulation, STZ-injected rats presented significantly higher numbers of Fos-IR neurons than controls, both in the superficial and deep dorsal horn (DDH). In comparison with the respective baseline levels, innocuous stimulation did not induce a significant increase in the numbers of Fos-IR neurons in controls or STZ-rats. Noxious mechanical and noxious thermal stimuli increased the numbers of Fos-IR neurons, both in control and STZ-rats, but in a more pronounced manner when diabetic rats were subjected to noxious mechanical stimulation.

CONCLUSIONS

The present study demonstrates that the responses of spinal cord neurons are strongly affected during diabetes. The higher baseline neuronal activity probably underlies the spontaneous pain detected during diabetes since the spinal dorsal horn is the major relay station in the ascending transmission of nociceptive input to the brain.

Inflammation-induced enhancement of the visceromotor reflex to urinary bladder distention: modulation by endogenous opioids and the effects of early-in-life experience with bladder inflammation

Abdominal electromyographic (EMG) responses to noxious intensities of urinary bladder distention (UBD) are significantly enhanced 24 hours after zymosan-induced bladder inflammation in adult female rats. This inflammation-induced hypersensitivity is concomitantly inhibited by endogenous opioids because intraperitoneal (i.p.) naloxone administration before testing significantly increases EMG response magnitude to UBD. This inhibitory mechanism is not tonically active because naloxone does not alter EMG response magnitude to UBD in rats without inflammation. At the dose tested, naloxone does not affect bladder compliance in rats with or without inflammation. The effects of i.p. naloxone probably result from blockade of a spinal mechanism because intrathecal naloxone also significantly enhances EMG responses to UBD in rats with inflammation. Rats exposed to bladder inflammation from P90-P92 before reinflammation at P120 show similar hypersensitivity and concomitant opioid inhibition, with response magnitudes being no different from that produced by inflammation at P120 alone. In contrast, rats exposed to bladder inflammation from P14-P16 before reinflammation at P120 show markedly enhanced hypersensitivity and no evidence of concomitant opioid inhibition. These data indicate that bladder inflammation in adult rats induces bladder hypersensitivity that is inhibited by an endogenous opioidergic mechanism. This mechanism can be disrupted by neonatal bladder inflammation.

PERSPECTIVE

The present study observed that bladder hypersensitivity resulting from acute bladder inflammation is suppressed by an opioid-inhibitory mechanism. Experiencing bladder inflammation during the neonatal period can impair the expression of this opioid inhibitory mechanism in adulthood. This suggests that bladder insults during development may permanently alter visceral sensory systems and may represent 1 cause of painful bladder disorders.

Differential and synergistic effects of selective norepinephrine and serotonin reuptake inhibitors in rodent models of pain

There is increasing recognition that norepinephrine (NE) and serotonin (5-HT) reuptake inhibitors (NRIs and SRIs) are efficacious in treating some types of pain. To date, studies have not systematically evaluated the relative activity at the NE and/or 5-HT transporter required for maximal efficacy in rodent pain models. Known selective NE and 5-HT reuptake inhibitors reboxetine, desipramine, fluoxetine, and paroxetine were evaluated in both in vitro and in vivo assays. Using the spinal nerve ligation model of neuropathic pain, the compounds differentially reversed tactile allodynia. Evaluation of a broader spectrum of reuptake inhibitors in the para-phenylquinone (PPQ)-induced abdominal constriction model, a model of acute visceral pain, demonstrated that both the SRIs and the NRIs significantly blocked abdominal constrictions. However, the magnitude of this effect was greater following treatment with compounds having greater affinity for NRI compared with SRI affinity. In addition, isobolographic analyses indicated significant synergistic effects for all combinations of desipramine and fluoxetine in the PPQ model of visceral pain. Collectively, the present results support the hypothesis that compounds with greater NRI activity should be more effective for the treatment of pain than compounds having only SRI activity, and this hypothesis is also supported by clinical data. These studies also suggest that the potency and effectiveness of NRIs might be enhanced by the presence of 5-HT activity.

Descending facilitation from the rostral ventromedial medulla maintains nerve injury-induced central sensitization

Nerve injury can produce hypersensitivity to noxious and normally innocuous stimulation. Injury-induced central (i.e. spinal) sensitization is thought to arise from enhanced afferent input to the spinal cord and to be critical for expression of behavioral hypersensitivity. Descending facilitatory influences from the rostral ventromedial medulla have been suggested to also be critical for the maintenance, though not the initiation, of experimental neuropathic pain. The possibility that descending facilitation from the rostral ventromedial medulla is required for the maintenance of central sensitization was examined by determining whether ablation of mu-opioid receptor-expressing cells within the rostral ventromedial medulla prevented the enhanced expression of repetitive touch-evoked FOS within the spinal cord of animals with spinal nerve ligation injury as well as nerve injury-induced behavioral hypersensitivity. Rats received a single microinjection of vehicle, saporin, dermorphin or dermorphin-saporin into the rostral ventromedial medulla and 28 days later, underwent either sham or spinal nerve ligation procedures. Animals receiving rostral ventromedial medulla pretreatment with vehicle, dermorphin or saporin that were subjected to spinal nerve ligation demonstrated both thermal and tactile hypersensitivity, and showed significantly increased expression of touch-evoked FOS in the dorsal horn ipsilateral to nerve injury compared with sham-operated controls at days 3, 5 or 10 post-spinal nerve ligation. In contrast, nerve-injured animals pretreated with dermorphin-saporin showed enhanced behaviors and touch-evoked FOS expression in the spinal dorsal horn at day 3, but not days 5 and 10, post-spinal nerve ligation when compared with sham-operated controls. These results indicate the presence of nerve injury-induced behavioral hypersensitivity associated with nerve injury-induced central sensitization. Further, the results demonstrate the novel concept that once initiated, maintenance of nerve injury-induced central sensitization in the spinal dorsal horn requires descending pain facilitation mechanisms arising from the rostral ventromedial medulla.

Expression of opioid receptors and c-fos in CB1 knockout mice exposed to neuropathic pain

The development of neuropathic pain is associated with multiple changes in gene expression occurring in the dorsal root ganglia (DRG) and spinal cord. The goal of this study was to evaluate whether the disruption of CB1 cannabinoid receptor gene modulates the changes induced by neuropathic pain in the expression of mu- (MOR), delta- (DOR) and kappa-opioid receptors (KOR) mRNA levels in the DRG and spinal cord. The induction of c-fos expression in the lumbar and sacral regions of the spinal cord was also evaluated in these animals. Opioid receptors mRNA levels were determined by using real-time PCR and Fos protein levels by immunohistochemistry. Nerve injury significantly reduced the expression of MOR in the DRG and the lumbar section of the spinal cord from CB1 cannabinoid knockout (KO) mice and wild-type littermates (WT). In contrast, mRNA levels of DOR and KOR were not significantly changed in any of the different sections analysed. Furthermore, sciatic nerve injury evoked a similar increase of c-fos expression in lumbar and sacral regions of the spinal cord of both KO and WT. In all instances, no significant differences were observed between WT and KO mice. These data revealed specific changes induced by neuropathic pain in MOR expression and c-fos levels in the DRG and/or spinal cord that were not modified by the genetic disruption of CB1 cannabinoid receptors.

Persistent monoarthritis of the temporomandibular joint region enhances nocifensive behavior and lumbar spinal Fos expression after noxious stimulation to the hindpaw in rats

Effects of persistent temporomandibular joint (TMJ) inflammation on nociceptive responses of remote bodily areas of the rat were investigated. Monoarthritis of the TMJ region was evoked by the injection of complete Freund's adjuvant (CFA) into the left TMJ region. Rats without injection of CFA into the TMJ region served as controls (non-CFA group). Time spent on licking behavior evoked by the injection of formalin into the left hindpaw and withdrawal thresholds of mechanical stimulation to both sides of the hindpaw were measured during TMJ inflammation for 3 weeks. Furthermore, expression of Fos protein in the lumbar dorsal horn was immunohistochemically investigated following the injection of formalin into the hindpaw during TMJ inflammation. Formalin-evoked nocifensive behavioral activities were significantly enhanced at 10 and 14 days after CFA injection in the late phase, while the withdrawal threshold to mechanical stimulation was significantly decreased bilaterally at 8, 10 and 14 days after CFA injection. Both formalin-evoked licking behavior and mechanical withdrawal thresholds to bilateral hindpaw at 21 days after CFA injection were similar to those in the non-CFA group. The number of Fos-positive neurons in the lumbar dorsal horn ipsilateral to the formalin injection at 1 and 7 days after CFA injection into the TMJ were similar to those in the non-CFA group; however, those were significantly increased in the laminae I-II and V-VI of the lumbar dorsal horn at 14 days after CFA injection. TMJ inflammation for 7 and 14 days alone produced a small number of Fos-expressing neurons in the lumbar dorsal horn. These results provide evidence that persistent unilateral inflammation of the TMJ region causes an increase in behavioral hyperalgesia of the hindpaw, which is attributed to the modulation of neural activities, in part, in the lumbar dorsal horn, likely mediated by supraspinal neural mechanisms.

Effect of nor-trimebutine on neuronal activation induced by a noxious stimulus or an acute colonic inflammation in the rat

Nor-trimebutine is the main metabolite of trimebutine that is used in the treatment of patients with irritable bowel syndrome. Nor-trimebutine has a blocking activity on sodium channels and a potent local anesthetic effect. These properties were used to investigate the effect of nor-trimebutine on spinal neuronal activation induced by models of noxious somato-visceral stimulus and acute colonic inflammation. Nor-trimebutine was administered in rats either subcutaneously 30 min before intraperitoneal administration of acetic acid or intracolonically 30 min before intracolonic infusion of trinitrobenzenesulfonic acid. Abdominal contractions were counted for 1 h as a marker of abdominal pain. c-fos expression was used as a marker of neuronal activation and revealed by immunohistochemistry 1h after intraperitoneal acetic acid injection and 2 h after colonic inflammation. Nor-trimebutine decreased Fos expression in the thoraco-lumbar (peritoneal irritation) and lumbo-sacral (colonic inflammation) spinal cord in laminae I, IIo V, VII and X. This effect was also observed in the sacral parasympathetic nucleus after colonic inflammation. Nor-trimebutine induced a significant decrease of abdominal contractions following intraperitoneal acetic acid injection. These data may explain the effectiveness of trimebutine in the therapy of abdominal pain in the irritable bowel syndrome.

The physiology and processing of pain: a review

Despite the many advances in our understanding of the mechanisms underlying pain processing, pain continues to be a major healthcare problem in the United States. Each day, millions of Americans are affected by both acute and chronic pain conditions, costing in excess of $100 billion for treatment-related costs and lost work productivity. Thus, it is imperative that better treatment strategies be developed. One step toward improving pain management is through increased knowledge of pain physiology. Within the nervous system, there are several pathways that transmit information about pain from the periphery to the brain. There is also a network of pathways that carry modulatory signals from the brain and brainstem that alter the incoming flow of pain information. This article provides a review to the physiology and processing of pain.

Effects of an acetone extract of Boswellia carterii Birdw. (Burseraceae) gum resin on rats with persistent inflammation

OBJECTIVE

Ruxiang, or Gummi olibanum, an herbal medicine derived from the gum resin of Boswellia carterii Birdw. (BC) of the family Burseraceae, has been used traditionally in China to alleviate pain and reduce inflammation. The present study is an investigation of the effects of a BC extract on persistent hyperalgesia and edema in rats with peripheral inflammation.

DESIGN

In this randomized, blinded study, the antihyperalgesic and antiedema effects of 3 dosages of BC were compared to a vehicle control. Inflammation was induced in rats by injecting complete Freund's adjuvant (CFA) into one hind paw. A single oral dose of the BC extract was administered daily for 7 days, beginning one day before CFA. Hyperalgesia was assessed using a paw withdrawal latency (PWL) test pre-CFA and 2 hours, 5 hours, 1 day, and 5 days post-CFA. Edema was determined by measuring paw thickness at the same time points. Spinal Fos protein expression was analyzed 2 hours post-CFA. Adverse effects of the extract were monitored by observing the animals closely for unusual behavioral changes.

RESULTS

Compared to control, a dosage of 0.45 g/kg BC significantly lengthened PWL and reduced paw edema on day 5 post-CFA. At 0.90 g/kg, BC significantly lengthened PWL at 5 hours, 1 day, and 5 days, and reduced paw edema at 2 hours, 5 hours, 1 day, and 5 days. This dosage also significantly suppressed spinal Fos expression in the medial half of laminae I-II. At 1.80 g/kg, BC significantly lengthened PWL and reduced paw edema at all time points. No noticeable adverse effects were observed in animals given the lower dosages of BC, but adverse effects in some animals were observed at 1.80 g/kg per day. In the acute toxicity study, the maximal single dose of 2.50 g/kg produced no adverse effects in the treated rats during the 14 days of observation.

CONCLUSIONS

The data suggest that BC produces significant antihyperalgesia and anti-inflammation effects and that the antihyperalgesia may be mediated by suppressed inflammation-induced Fos expression in the spinal dorsal horn neurons.

Effect of Chronic Inflammation on Dorsal Horn Nociceptive Neurons in Aged Rats

To elucidate the effect of chronic inflammation on spinal nociceptive neurons in the elderly, we compared nocifensive behavior, peripheral inflammatory responses, and spinal dorsal horn neuronal activities between the aged (29–34 mo) and adult (7–12 mo) male rats after injection of complete Freund's adjuvant (CFA) into the hind paw. Aged rats exhibited a significantly lower mechanical paw withdrawal threshold before inflammation. However, after CFA injection mechanical allodynia developed in both adult and aged rats after CFA injection. The changes of foot temperature and thickness after CFA injection were greater and lasted longer in aged than in adult rats. Sets of 124 wide dynamic range (WDR) neurons (aged: 59, adult: 65) and 26 nociceptive specific (NS) neurons (aged: 13, adult: 13) were recorded from the lumber spinal dorsal horn. NS neurons from the inflamed adult rats showed significantly higher responses to noxious mechanical stimulation than those in aged rats, whereas WDR neurons from inflamed adult and aged rats were similar. Background activity of WDR neurons from the adult rats increased after CFA, whereas WDR neurons of aged rats and NS neurons from either group were not. The afterdischarge followed by noxious mechanical stimulation was significantly greater for WDR neurons in both adult and aged rats, whereas no significant differences were observed in NS neurons. Two days after CFA injection, Fos expression increased similarly in aged and adult rats. Thus the aged rats showed enhanced peripheral inflammatory responses to CFA injection with only a slight change in dorsal horn neuronal activity. Together with our previous finding that nociceptive neurons in aged rats exhibit hyperexcitability, these results suggest that the dorsal horn nociceptive system becomes sensitized with advancing age and its excitability cannot be further increased by inflammation.

Commissural propriospinal connections between the lateral aspects of laminae III-IV in the lumbar spinal cord of rats

It has been established that there is a strong functional link between sensory neural circuits on the two sides of the spinal cord. In one of our recent studies we provided a morphological confirmation of this functional phenomenon, presenting evidence for the presence of a direct commissural connection between the lateral aspects of the dorsal horn on the two sides of the lumbar spinal cord. By using a combination of neural tracing and immunocytochemical detection of neural markers like vesicular glutamate transporters, glutamic acid decarboxylase, glycine transporter, and met-enkephalin (which are characteristic of various subsets of excitatory and inhibitory neurons), we investigated here the distribution, synaptic relations, and neurochemical characteristics of the commissural axon terminals. We found that the cells of origin of commissural fibers in the lateral aspect of the dorsal horn were confined to laminae III-IV and projected to the corresponding area of the contralateral gray matter. Most of the commissural axon terminals established synaptic contacts with dendrites. Axospinous or axosomatic synaptic contacts were found in limited numbers. We demonstrated that interactions among commissural neurons also exist. More than three-fourths of the labeled axon terminals were immunostained for glutamic acid decarboxylase and/or glycine transporter, but none of them showed positive immunoreaction for met-enkephalin and vesicular glutamate transporters. The results indicate that there is a substantial reciprocal commissural synaptic interaction between the lateral aspects of laminae III-IV on the two sides of the lumbar spinal cord and that this pathway may transmit both inhibitory and excitatory signals to their postsynaptic targets.

The caudal ventrolateral medulla as an important inhibitory modulator of pain transmission in the spinal cord

The caudal ventrolateral medulla (VLM) has emerged during the last decade as one of the main components of the endogenous pain control system. Profound and long-lasting analgesia is produced by mild stimulation of the VLM. The VLMlat, the reticular formation located between the spinal trigeminal nucleus and the lateral reticular nucleus (LRt), appears to play a major role in that antinociceptive action. The projections to spinal cord laminae involved in nociceptive transmission originate exclusively in the VLMlat. The VLMlat participates in a disynaptic pathway involving spinally projecting pontine A5 noradrenergic neurons, which appears to convey alpha(2)-adrenoreceptor-mediated analgesia produced from the VLM. Neurons in the VLMlat and in lamina I are reciprocally connected by a closed loop that is likely to mediate feedback control of supraspinal nociceptive transmission. On the other hand, the LRt, which is targeted by ventral (lamina VII) and deep dorsal (laminae IV to V) horn inputs, projects to the premotor lamina VII. Nociceptive input ascending from the cord and increases in blood pressure are discussed as possible physiologic triggers of the analgesia produced by the VLM. The overall role of the VLM as a center for integration of nociceptive, cardiovascular, and motor functions is discussed. The putative therapeutic benefits of manipulating the VLM for the control of chronic pain are envisaged.

Unilateral hindpaw inflammation induces bilateral activation of the locus coeruleus and the nucleus subcoeruleus in the rat

Several lines of evidence have shown that unilateral hindpaw inflammation produces activation of the locus coeruleus (LC) and the nucleus subcoeruleus (SC), resulting in descending modulation of nociceptive processing in the dorsal horn. However, it is unclear if the LC/SC is activated unilaterally or bilaterally following the development of unilateral hindpaw inflammation. The present study was designed to clarify this question. For the induction of unilateral hindpaw inflammation, lambda carrageenan (2.0mg in 0.15ml saline) was injected subcutaneously into the plantar surface of the left hindpaw. Four hours after carrageenan injection, in the LC/SC both ipsilateral and contralateral to the inflamed paw, the number of Fos-positive cells increased significantly in carrageenan-injected rats when compared to vehicle (saline)-injected and untreated control rats. The Fos expression in the LC/SC was equivalent bilaterally in the carrageenan-injected rats, as well as in vehicle-injected and untreated control rats. For nociceptive testing, the paw withdrawal latency, which measures cutaneous hyperalgesia in response to thermal stimuli, was determined in rats receiving a unilateral lesion of the LC/SC either ipsilateral or contralateral to the inflamed paw. Two and a half hours after the induction of inflammation, in both groups of rats with unilateral lesion, paw withdrawal latencies decreased significantly in the LC/SC-lesioned rats. However, there was no significant difference in paw withdrawal latencies between the LC/SC-lesioned rats and sham-operated rats, indicating that unilateral activation of the LC/SC is sufficient for modulating nociceptive processing in the dorsal horn. These results suggest that unilateral hindpaw inflammation induces bilateral activation of the LC/SC.

Involvement of the anterior pretectal nucleus in the control of persistent pain: a behavioral and c-Fos expression study in the rat

The anterior pretectal nucleus (APtN) participates in nociceptive processing and in the activation of central descending mechanisms of pain control. In this study we used behavioral tests (incisional pain and carrageenan-induced inflammatory pain) and c-Fos expression changes to examine the involvement of the APtN in the control of persistent pain in rats. A 1cm longitudinal incision through the skin and fascia of the plantar region (large incision), or a 0.5cm longitudinal incision through the skin only (small incision) was used, and the postoperative incisional allodynia was evaluated with von Frey filaments. The hyperalgesia produced by the intraplantar administration of carrageenan (25 or 50 microg/100 microl) into a hind paw was evaluated by a modified paw pressure test. The electrolytic lesion of the contralateral, but not ipsilateral, APtN significantly intensified the allodynia produced by a large incision of the hind paw. The incisional allodynia and the carrageenan-induced hyperalgesia were intensified by the microinjection of 2% lidocaine into the contralateral, but not ipsilateral APtN, the effect being significantly stronger when a large incision or a higher carrageenan concentration was utilized. A significant increase in the number of c-Fos positive cells was found in the ipsilateral, and mainly in the contralateral APtN of rats submitted to a large incision. The number of positive cells in the superficial or deep laminae of the contralateral spinal cord of control and incised rats was not significantly different. Positive cells in the superficial or deep laminae of the ipsilateral spinal cord were significantly more numerous than in control, the effect being significantly more intense in rats with large incision. The microinjection of 0.5% bupivacaine into the APtN contralateral to the incised hind paw reduced the number of positive cells bilaterally in the APtN, but the effect was significant in the contralateral nucleus only. The number of positive cells in the superficial and deep laminae of the contralateral spinal cord of incised and non-incised animals was not significantly changed by the neural block of the contralateral APtN. In the ipsilateral spinal cord, the incision-induced increase in the number of positive cells was significantly reduced in the superficial lamina and significantly increased in the deep lamina of animals previously treated with bupivacaine in the contralateral APtN. In conclusion, the integrity of the APtN is necessary to reduce the severity of the responses to persistent injury. The results also are in agreement with the current notion that persistent noxious inputs to the APtN tonically activate a descending mechanism that excites superficial cells and inhibits deep cells in the spinal dorsal horn.

Strain-dependent modification of neuropathic pain behaviour in the rat hindpaw by a priming painful trigeminal nerve injury

The aim of the present study was to test the behavioural effect of infraorbital (IO) chronic constriction injury (CCI) on the development of neuropathic pain in the rat hindpaw following sciatic nerve CCI performed 7 days later. Control groups consisted of rats that underwent sham infraorbital surgery followed by sciatic CCI at identical time points. Sensory testing of the rat's face and hindpaw was performed at baseline and at 4, 11, 14, 17 and 21 days postoperative (dpo) relative to the IOCCI, at which time all rats were euthanized. To test for strain differences the experiment was performed on Sprague-Dawley, Sabra and Lewis rats. In Lewis rats the trigeminal nerve injury significantly accelerated the development of hindpaw mechanoallodynia (11th, 14th and 17th dpo, unpaired t-test, P<0.05) and mechanohyperalgesia (14th and 17th dpo, unpaired t-test, P<0.05), following a second sciatic nerve CCI relative to the control group. This effect was not observed in Sprague-Dawley or Sabra rats.

The roles of NMDA receptor activation and nucleus reticularis gigantocellularis in the time-dependent changes in descending inhibition after inflammation

Previous studies indicate that descending modulation of nociception is progressively increased following persistent inflammation. The present study was designed to further examine the role of supraspinal neurons in descending modulation following persistent inflammation. Constant levels of paw withdrawal (PW) and tail flick (TF) latencies to noxious heat stimuli were achieved in lightly anesthetized rats (pentobarbital sodium 3-10 mg/kg/h, i.v.). Electrical stimulation (ES, 0.1 ms, 100 Hz, 20-200 A) was delivered to the rostral ventromedial medulla (RVM), mainly the nucleus raphe magnus (NRM). ES produced intensity-dependent inhibition of PW and TF. Following a unilateral hindpaw inflammation produced by injection of complete Freund's adjuvant (CFA), ES-produced inhibition underwent time-dependent changes. There was an initial decrease at 3 h after inflammation and a subsequent increase after inflammation in the excitability of RVM neurons and the inhibition of nocifensive responses. These changes were most robust after stimulation of the inflamed paw although similar findings were seen on the non-inflamed paw and tail. The inflammation-induced dynamic changes in descending modulation appeared to be correlated with changes in the activation of the N-methyl--aspartate (NMDA) excitatory amino acid receptor. Microinjection of an NMDA receptor antagonist, AP5 (1 pmol), resulted in an increase in the current intensity required for inhibition of the PW and TF. The effect of AP5 was less at 3 h after inflammation and significantly greater at 11-24 h after inflammation. In a subsequent experiment, ES-produced inhibition of nocifensive responses after inflammation was examined following selective chemical lesions of the nuclei reticularis gigantocellularis (NGC). Compared to vehicle-injected animals, microinjection of a soma-selective excitotoxin, ibotenic acid, enhanced ES-produced inhibition at 3 h but not at 24 h after inflammation. We propose that these time course changes reflect dynamic alterations in concomitant descending facilitation and inhibition. At early time points, NMDA receptor and NGC activation enhance descending facilitation; as time progresses, the dose-response curve of NMDA shifts to the left and descending inhibition dominates and masks any descending facilitation.

A novel analgesic compound OT-7100 attenuates nociceptive responses in animal models of inflammatory and neuropathic hyperalgesia: a possible involvement of adenosinergic anti-nociception

We studied the effects of OT-7100 (5-n-butyl-7-(3,4,5-trimethoxybenzoylamino)pyrazolo [1,5-a]pyrimidine), a novel analgesic compound, on the inhibitory action of adenosine on the contraction of guinea pig ileum and investigated the effects of OT-7100 on the nociceptive responses in animal models of inflammatory and peripheral neuropathic hyperalgesia and decreases spinal c-Fos expression. OT-7100 at 0.3 - 3 microM significantly enhanced the inhibitory effect of adenosine on the contraction of guinea pig ileum. The efficacy of OT-7100 (1, 3 or 10 mg/kg, p.o.) on hyperalgesia induced by yeast or substance P and in the Bennett model was significantly suppressed by coadministration of the adenosine A1 antagonist DPCPX (0.01 or 0.1 pmol/animal, i.t.), while OT-7100 without DPCPX significantly increased the nociceptive threshold in each rat model. OT-7100 (3, 10 and 30 mg/kg per day, p.o.) significantly inhibited the mechanical nociceptive threshold in the injured paw in the Chung model. OT-7100 (30 mg/kg, p.o.) significantly decreased the number of Fos-LI neurons in the spinal dorsal horn in the Bennett model. These finding suggest that OT-7100 inhibits hyperalgesia in these animal models possibly by enhancing adenosinergic neurotransmission in the dorsal horn, although we still lack direct evidence for it.

Hyperalgesia induced by peripheral inflammation is mediated by protein kinase C betaII isozyme in the rat spinal cord

We have addressed the molecular mechanism(s) of hyperalgesia, which depends on increased excitability of dorsal horn neurons and on sensitization of primary afferent nociceptors, during peripheral inflammation. Following unilateral adjuvant-induced inflammation in the rat hind paw, time-course changes in behavioral hyperalgesia and functional activities of Ca2+/phospholipid-dependent protein kinase C isozymes were examined. Inflammation was characterized by increase in paw diameter, and behavioral hyperalgesia was quantified as paw withdrawal latency from a radiant heat source. Behavioral hyperalgesia on the injected paw was significantly increased. This was accompanied by a significant increase in total functional membrane-associated protein kinase C activity, whereas total cytosolic protein kinase C activity was unchanged on the sides of the lumbar spinal cord both contralateral and ipsilateral to the inflammation. Importantly, on the side of lumbar cord ipsilateral to the inflamed paw, the activity of membrane-associated protein kinase CbetaII was increased following the same time-course as the paw withdrawal latency decrease, suggesting an increased translocation of protein kinase Cbetall to the membrane related to behavioral hyperalgesia. A defined mixture of purified gangliosides, which inhibits intracellular protein kinase C translocation and activation, decreased inflammation-induced paw withdrawal latency, and specifically decreased the activity of membrane-associated protein kinase Cbetall on the side of the spinal cord ipsilateral to the inflammation. Quantitative immunohistochemical analyses demonstrated intensified protein kinase CbetaII-like immunoreactivity on the side of the spinal cord ipsilateral to the inflammation. Time-course for increases in the activity of membrane-associated protein kinase CbetaII, and in intensity of protein kinase CbetaII-immunoreactivity, paralleled inflammation-mediated changes in paw withdrawal latency and paw diameter. Our findings indicate an apparent involvement of protein kinase CbetaII isozyme specifically in the molecular mechanism(s) of thermal hyperalgesia.

The role of 5-hydroxytryptamine1A and 5-hydroxytryptamine1B receptors in modulating spinal nociceptive transmission in normal and carrageenan-injected rats

Single unit extracellular recordings from the dorsal horn neurons were obtained with glass micropipettes in pentobarbital-anesthetized rats. A total of 115 wide dynamic range (WDR) neurons were studied in 94 rats. In normal rats, the size of nociceptive receptive fields (RFs) of WDR neurons was approximately 123.3 +/- 8.21 mm2 (n = 88). Following carrageenan-induced inflammation, the RFs were markedly enlarged (332.4 +/- 30.1 mm2, n = 27, P < 0.001). The frequency of background activity of the WDR neurons in carrageenan-injected rats (11.3 +/- 2.1 imp/s, n = 27) was greater than that in normal rats (7.1 +/- 0.8 imp/s, n = 88, P < 0.05). In 82% of WDR neurons in normal rats, there was a separation between the A- and C-responses. In contrast, in 67% of the neurons in carrageenan-injected rats, the response to suprathreshold electrical stimuli was a long train with no separation between the A- and C-responses. In carrageenan-injected rats, the magnitude and duration of the nociceptive responses were significantly increased compared to those in normal rats, and the average C-response threshold (7.7 +/- 1.1 mA, n = 27) was lower than that in normal rats (10.4 +/- 0.7 mA, n = 88, P < 0.05). Intrathecal injection of the 5-hydroxytryptamine(1A) (5-HT1A) receptor agonist 8-hydroxy-DPAT hydroxybromide (8-OH-DPAT) (0.305, 1.525, 3.05, and 15.25 mM) dose-dependently increased Adelta- and C-responses and post-discharge in most of the WDR neurons. Following carrageenan-induced inflammation, the 8-OH-DPAT-induced facilitatory effect on Adelta- and C-responses and post-discharge was significantly enhanced (P < 0.05). Intrathecal injection of the 5-hydroxytryptamine1B (5-HT1) receptor agonist CGS12066A (0.222, 1.11, 2.22, and 11.1 mM) dose-dependently enhanced the C-response and post-discharge without influencing the Adelta-response. In carrageenan-injected rats, CGS12066A not only enhanced the facilitatory effect on the C-response and post-discharge, but also facilitated the Adelta-response. Intrathecal injection of the 5-HT(1A) receptor antagonist NAN-190 (0.2 mM) alone did not influence Adelta- and C-responses and post-discharge of WDR neurons in normal rats. When 0.2 mM NAN-190 was co-administered with 3.05 mM 8-OH-DPAT, the facilitatory effect of 8-OH-DPAT on Adelta- and C-responses and post-discharge was completely antagonized, whereas CGS12066A-induced facilitation on the C-response and post-discharge was not influenced by co-administration of 0.2 mM NAN-190 and CGS12066A. These data suggest that 5-HT1A and 5-HT1B receptor subtypes mediate the facilitatory effect of 5-HT on nociceptive processing in the spinal cord of rats. The excitability of dorsal horn WDR neurons and the sensitivity of the neurons to intrathecal 5-HT1A and 5-HT1B receptor agonists might increase following carrageenan-induced inflammation.

Increased Fos induction in adult rats that experienced neonatal peripheral inflammation

The response to noxious stimulation was compared in adult rats that had peripheral inflammation as neonates and untreated rats. On postnatal day 1, rat pups experienced complete Freund's adjuvant (CFA)-induced inflammation of the left hind paw. At 8 weeks of age, these rats and neonatal untreated rats received a bilateral injection of CFA into their hind paws. Fos-like immunoreactivity (Fos-LI) was used as a measure of neuronal activity in dorsal horn nociceptive pathways. A significant increase in Fos-LI was found on the left side of the lumbar spinal cord of neonatal treated rats as compared to neonatal untreated rats. These results suggest that the experience of neonatal peripheral inflammation may result in an increase in the response of spinal cord neurons to peripheral inflammation as adults.

Stage-dependent changes in the modulation of spinal nociceptive neuronal activity during the course of inflammation

Spinal and supraspinal controls can tonically or phasically modulate the output of spinal nociceptive neurons. Alterations of these modulatory systems have been described during the acute stage of inflammation. In the present study in the rat, tonic descending controls were assessed during acute (24--48 h) and chronic (3--4 weeks) stages of monoarthritis of the ankle. The electrophysiological properties of spinal convergent neurons with ankle input were compared before and after spinalization. In a parallel series of experiments, spinal convergent neurons were recorded from the normal side in order to assess the propriospinal and supraspinal inhibitory controls triggered by nociceptive stimulation of the inflamed ankle. Tonic descending inhibition of convergent neurons with input from the inflamed ankle was enhanced during the acute stage and then decreased during the chronic stage of monoarthritis. Contralateral-induced inhibitions exhibited a similar temporal evolution. Time-dependent changes in the spinal transmission of nociceptive signals were shown by removing descending modulation in animals with monoarthritis; sensitization of spinal neurons with input from the inflamed ankle was demonstrated during the acute stage of monoarthritis, whereas a crossed transmission between inflamed and normal sides was observed during the chronic stage of the disease. These results show that dynamic and stage-dependent modifications of descending controls tend to dampen the central changes associated with inflammation.

Propriospinal afferent and efferent connections of the lateral and medial areas of the dorsal horn (laminae I-IV) in the rat lumbar spinal cord

The different subdivisions along the mediolateral extent of the superficial dorsal horn of the spinal cord are generally regarded as identical structures that execute the function of sensory information processing without any significant communication with other regions of the spinal gray matter. In contrast to this standing, here we endeavor to show that neural assemblies along the mediolateral extent of laminae I-IV cannot be regarded as identical structures. After injecting Phaseolus vulgaris leucoagglutinin and biotinylated dextran amine into various areas of the superficial dorsal horn (laminae I-IV) at the level of the lumbar spinal cord in rats, we have demonstrated that the medial and lateral areas of the superficial dorsal horn show the following distinct features in their propriospinal afferent and efferent connections: 1) A 300- to 400-microm-long section of the medial aspects of laminae I-IV projects to and receives afferent fibers from a three segment long compartment of the spinal dorsal gray matter, whereas the same length of the lateral aspects of laminae I-IV projects to and receives afferent fibers from the entire rostrocaudal extent of the lumbar spinal cord. 2) The medial aspects of laminae I-IV project extensively to the lateral areas of the superficial dorsal horn. In contrast to this, the lateral areas of laminae I-IV, with the exception of a few fibers at the segmental level, do not project back to the medial territories. 3) There is a substantial direct commissural connection between the lateral aspects of laminae I-IV on the two sides of the lumbar spinal cord. The medial part of laminae I-IV, however, does not establish any direct connection with the gray matter on the opposite side. 4) The lateral aspects of laminae I-IV appear to be the primary source of fibers projecting to the ipsi- and contralateral ventral horns and supraspinal brain centers. Projecting fibers arise from the medial subdivision of laminae I-IV in a substantially lower number. The findings indicate that the medial and lateral areas of the superficial spinal dorsal horn of rats may play different roles in sensory information processing.

Chronic tooth pulp inflammation causes transient and persistent expression of Fos in dynorphin-rich regions of rat brainstem

We have analyzed central Fos immunoreactivity (Fos-IR) brainstems of adult rats after three clinically relevant dental injuries: filled dentin (DF) cavities that cause mild pulp injury and heal within 1-2 weeks; open pulp exposures (PX) that cause gradual pulp loss and subsequent periodontal lesions; and filled pulp exposures (PXF). By 1 week after DF cavities, no Fos-IR remained except for sites such as lateral-ventral periolivary nucleus (LVPO) that had Fos-IR in all rats including controls. PX injury induced (1) a delayed transient expression of Fos at 1-2 weeks at three loci (ipsilateral neurons in dorsomedial nucleus oralis, paratrigeminal nucleus, and trigeminal tract), (2) persistent ipsilateral Fos for at least 4 weeks after injury in dynorphin (Dyn)-rich regions (rostral lateral solitary nucleus, periobex dorsal nucleus caudalis), and (3) late Fos-IR at 2-4 weeks (bilateral superficial cervical dorsal horn, contralateral dorsal nucleus caudalis, contralateral rostral lateral solitary nucleus). Rats with PXF injury were examined at 2 weeks, and they had greater numbers and more extensive rostro-caudal distribution of Fos neurons than the PX group. One week after PX injury, Fos-IR neurons were found in regions with strong Dyn-IR central fibers. Co-expression of Dyn and Fos was found in some unusually large neurons of the ipsilateral rostral lateral solitary nucleus, trigeminal tract, and dorsal nucleus caudalis. Immunocytochemistry for the p75 low affinity neurotrophin receptor (p75NTR) or for calcitonin gene-related peptide (CGRP) showed no consistent change in trigeminal central endings in any Fos-reactive brainstem areas, despite the extensive structural and cytochemical reorganization of the peripheral endings of the dental neurons. The Fos responses of central neurons to tooth injury have some unusual temporal and spatial patterns in adult rats compared to other trigeminal injury models.

Persistent Fos protein expression after orofacial deep or cutaneous tissue inflammation in rats: implications for persistent orofacial pain

This study was designed to systematically examine the effects of persistent orofacial tissue injury on prolonged neuronal activation in the trigeminal nociceptive pathways by directly comparing the effects of orofacial deep vs. cutaneous tissue inflammation on brainstem Fos protein expression, a marker of neuronal activation. Complete Freund's adjuvant (CFA) was injected unilaterally into the rat temporomandibular joint (TMJ) or perioral (PO) skin to produce inflammation in deep or cutaneous tissues, respectively. Rats were perfused 2 hours, 24 hours, 3 days, or 10 days following CFA injection. The TMJ and PO inflammation-induced Fos expression paralleled the intensity and course of inflammation over the 10-day observation period, suggesting that the increase in intensities and persistence of Fos protein expression may be associated with a maintained increase in peripheral input. Compared to PO CFA injection, the injection of CFA into the TMJ produced a significantly stronger inflammation associated with a greater Fos expression. In TMJ- but not in PO-inflamed rats, Fos-like immunoreactivity (LI) spread from superficial to deep upper cervical dorsal horn as the inflammation persisted and there was a dominant ipsilateral Fos-labeling in the paratrigeminal nucleus. Common to TMJ and PO inflammation, Fos-LI was induced in the trigeminal subnuclei interpolaris and caudalis, C1-2 dorsal horn, and other medullary nuclei. Substantial bilateral Fos-LI was found in the interpolaris-caudalis trigeminal transition zone. Further analysis revealed that Fos-LI in the ventral transition zone was equivalent bilaterally, whereas Fos-LI in the dorsal transition zone was predominantly ipsilateral to the inflammation. The differential induction of Fos expression suggests that an increase in TMJ C-fiber input after inflammation and robust central neuronal hyperexcitability contribute to persistent pain associated with temporomandibular disorders.

Endogenous mechanisms of sensory modulation

We provide evidence supporting the idea that the relationship between tissue damage, or the threat of tissue damage, and the response to such stimuli is variant and dependent on neuronal networks by which attentional, emotional and cognitive components of pain experience activate endogenous descending modulatory systems. Most previous studies have focused on responses to transient noxious stimuli with little information on the influence of descending modulation on behavioral responses to persistent pain and hyperalgesia after tissue or nerve injury. Utilizing correlative behavioral and neuronal studies we have demonstrated that (1) behavioral context modulates neuronal activity in nociceptive and non-nociceptive somatosensory pathways, supporting the hypothesis that responses in these pathways are not immutable; (2) descending modulation influences behavior and neuronal activity at spinal cord levels after inflammation and persistent pain; and (3) there are descending facilitatory as well as inhibitory influences on behavior and spinal cord neuronal activity that may impact on persistent pain particularly of deep muscle and visceral origin. Cortical as well as subcortical pathways are available by which dorsal horn activity can be modulated by attentional, motivational and cognitive factors. It appears that the same neuronal mechanisms in the forebrain and brain stem are available for behavioral modulation in a learned task involving the threat of tissue damage (transient noxious stimuli) as are available in the development and amplification of persistent pain produced by inflammation. These parallel brain mechanisms emphasize the saliency of pain experience as an important learned behavior for the survival of the organism, similar to sequential goal-directed behaviors in an operant task.

Laminar-selective noradrenergic and serotoninergic modulation includes spinoparabrachial cells after inflammation

We evaluated the effects of chemical lesions on hindpaw inflammation-induced Fos protein expression in spinoparabrachial neurons that were retrogradely labeled by Fluoro-Gold. The descending serotoninergic and noradrenergic pathways were destroyed by the selective neurotoxins, 5,7-DHT and DSP-4, respectively. After 5,7-DHT treatment there was a significant increase in double-labeled neurons only in the lateral reticulated neck of the dorsal horn 24h after inflammation compared with vehicle-injected controls. In contrast, the DSP-4 treatment resulted in a more robust increase in double-labeled neurons in the ipsilateral superficial dorsal horn than in the neck of the dorsal horn. These results indicate that after inflammation the enhanced modulation from descending serotoninergic and noradrenergic pathways targets supraspinally projecting neurons to dampen increased ascending nociceptive input. Further, these pathways differentially suppress the responses of spinoparabrachial neurons in the deep and superficial dorsal horn.

Inflammatory Models of Pain and Hyperalgesia

This article addresses important pain research models in nonhuman animals. These models attempt to mimic human persistent pain conditions. Models of persistent pain employ inflammatory agents that produce discomfort and hyperalgesia (i.e., an enhanced response to a noxious stimulus). The models are associated with skin, subcutaneous tissue, and joint inflammation (somatic structures). Studies employing such models have led to significantly improved understanding of mechanisms of somatic pain. It is important that investigators assess the level of pain produced in these animals and provide analgesic agents whenever it does not interfere with the purpose of the experiment. Pain can be inferred from ongoing behavioral variables such as feeding and drinking, sleep-waking cycle, grooming, and social behavior.

Inflammation-induced up-regulation of protein kinase Cgamma immunoreactivity in rat spinal cord correlates with enhanced nociceptive processing

Activation of various second messengers contributes to long-term changes in the excitability of dorsal horn neurons and to persistent pain conditions produced by injury. Here, we compared the time-course of decreased mechanical nociceptive thresholds and the density of protein kinase Cgamma immunoreactivity in the dorsal horn after injections of complete Freund's adjuvant in the plantar surface of the rat hindpaw. Complete Freund's adjuvant significantly increased paw diameter and mechanical sensitivity ipsilateral to the inflammation. The changes peaked one day post-injury, but endured for at least two weeks. In these rats, we recorded a 75-100% increase in protein kinase Cgamma immunoreactivity in the ipsilateral superficial dorsal horn of the L4 and L5 segments at all time-points. Electron microscopy revealed that the up-regulation was associated with a significant translocation of protein kinase Cgamma immunoreactivity to the plasma membrane. In double-label cytochemical studies, we found that about 20% of the protein kinase Cgamma-immunoreactive neurons, which are concentrated in inner lamina II, contain glutamate decarboxylase-67 messenger RNA, but none stain for parvalbumin or nitric oxide synthase. These results indicate that persistent changes in protein kinase Cgamma immunoreactivity parallel the time-course of mechanical allodynia and suggest that protein kinase Cgamma contributes to the maintenance of the allodynia produced by peripheral inflammation. The minimal expression of protein kinase Cgamma in presumed inhibitory neurons suggests that protein kinase Cgamma-mediated regulation of excitatory interneurons underlies the changes in spinal cord activity during persistent nociception.

Spinal section and opioid receptor blockade induce the appearance of Fos-like immunoreactivity in the spinal cord of the decerebrated rabbit

The expression of Fos-like immunoreactivity has been studied in spinal segments L5-S1 of decerebrated, unanaesthetized, but otherwise unstimulated rabbits. The aim of the study was to establish baseline levels of Fos in such preparations, and to examine how these might change after spinalization and opioid receptor blockade. In animals with an intact spinal cord, approximately 30 Fos-positive profiles per section were found in the superficial dorsal horns (i.e. laminae I and II) of each 40-microm section, while about 20 profiles per section were found immediately adjacent to the central canal (lamina X). Fos-like immunoreactive profiles were rare elsewhere in the gray matter. When the spinal cord was sectioned at L1 (after blockade with local anaesthetic), significantly more Fos-like immunoreactivity was found in superficial and central regions of the gray matter (approximately 90 profiles per section) in animals perfused 4 h after decerebration, but not when perfusion was performed 2 or 8 h after decerebration. The opioid antagonist naloxone (0.25 mg/kg/h) had little effect on expression of Fos-like immunoreactivity in spinalized preparations, but significantly increased the numbers of Fos-positive profiles in all but the ventral areas of the spinal gray matter in non-spinalized preparations. The present data show that spinal section induces a transient increase in expression of Fos in the superficial and central parts of the spinal gray matter. It appears that spinalization induces spontaneous activity in some neurons in these regions of the cord, presumably as a result of relief of descending inhibition. The effects of naloxone indicate that endogenous opioids exert tonic inhibition over Fos-expressing spinal neurons in non-spinalized rabbits.

Effect of post-injury NMDA antagonist treatment on long-term Fos expression and hyperalgesia in a model of chronic neuropathic pain

Chronic constriction injury (CCI) of the sciatic nerve results in persistent mechanical hyperalgesia together with Fos protein expression in the lumbar spinal cord. We have examined the relationship between mechanical hyperalgesia and Fos expression within the lumbar spinal cord on days 14, 35 and 55 after either CCI or sham operation. To determine the role of NMDA receptor mechanisms in the maintenance of hyperalgesia and Fos expression, the NMDA antagonist MK-801 (0.3 mg kg-1 s.c.) was administered daily on days 28 to 34 after operation. CCI animals developed unilateral hind limb hyperalgesia that persisted unchanged from days 14 to 55 of the study. MK-801 treatment reduced hyperalgesia by 57% (p=0.02) on day 35 in CCI animals but did influence hyperalgesia at day 55. In the spinal cord, Fos positive cells were present bilaterally throughout laminae 3-10 at all time points examined in both CCI and sham group animals. Fos counts ipsilateral to the side of injury in laminae 3-10 correlated significantly with hyperalgesia scores in the CCI but not sham animals. MK-801 treatment resulted in a suppression of Fos expression in ipsilateral laminae 3-4 (p=0.0017) and laminae 5-10 (p=0.0026) of CCI animals on day 35. Fos expression in sham group animals was not inhibited by MK-801 treatment at day 35. These results indicate that Fos expression is maintained by differing mechanisms following nerve injury or sham operation. The functional consequences of Fos expression following nerve injury and sham operation are discussed.

Effect of pre-emptive NMDA antagonist treatment on long-term Fos expression and hyperalgesia in a model of chronic neuropathic pain

The unilateral sciatic nerve chronic constriction injury (CCI) model of Bennett and Xie [G.J. Bennett, Y.-K. Xie, A peripheral neuropathy in rat that produces disorders of pain sensation like those seen in man, Pain, 33 (1988) 87-108] shows features of a neuropathic pain state. We examined mechanical hyperalgesia and Fos protein staining in the lumbar spinal cord 1, 7, 14 and 28 days after unilateral CCI to the sciatic nerve or sham operation. In addition, we examined the effect of the NMDA antagonist MK-801 (0.3 mg/kg s.c. administered 30 min prior to and 6 h following operation) on Fos expression and hyperalgesia at 28 days. CCI animals were hyperalgesic compared to the sham operated animals at 14 and 28 days post injury. MK-801 reduced hyperalgesia by 68% in CCI animals on day 28 (p=0.0001). In the spinal cord, Fos positive cells were present bilaterally in deeper laminae in both sham and CCI animals at all time points examined. Relatively few Fos positive cells were present in laminae 1-2 at any time point examined. At days 1 and 7, there were increased numbers of Fos positive cells ipsilaterally in the deeper laminae of the spinal cord in CCI animals compared to sham animals, but by 14 and 28 days Fos counts were similar in sham and CCI despite the obvious behavioural differences between the two groups. Fos counts ipsilateral to the injury in laminae 3-10 correlated with hyperalgesia scores in the CCI but not sham animals. Analysis at the 28-day time point showed that MK-801 differentially affected Fos expression: MK-801 significantly reduced the Fos count bilaterally in laminae 3-10 in the CCI but not in the sham group animals. These results indicate that Fos expression is initiated by different peripheral and central mechanisms following nerve injury or sham operation.

Dorsolateral funiculus-lesions unmask inhibitory or disfacilitatory mechanisms which modulate the effects of innocuous mechanical stimulation on spinal Fos expression after inflammation

To examine the contribution of low threshold mechanoreceptive afferent input to the development of allodynia and the involvement of descending pathways, we investigated the effects of repeated innocuous brush on inflammation-induced spinal Fos protein expression in dorsolateral funiculus-lesioned (DLFX) rats following hindpaw inflammation. In DLF sham-operated animals, brush stimuli induced a significant increase in the number of Fos-labeled neurons in ipsilateral laminae I-IV, and a slight suppression of Fos expression in ipsilateral laminae V-VI when compared to sham-lesioned rats without brushing. In rats receiving DLFX, the brush-induced increase in Fos expression in laminae I-IV was significantly reduced. The DLFX also unmasked a brush-induced suppression of laminae VII-VIII neurons. These results suggest that innocuous mechanical stimulation of an inflamed hindpaw gives rise to further facilitation of neuronal activity in laminae I-IV and inhibition of neuronal activity in laminae V-VIII. We propose that there is an unmasking of inhibitory mechanisms or a reduction in descending facilitatory effects after DLFX that alter Fos protein expression produced by innocuous mechanical stimulation.

The time-course of abeta-evoked c-fos expression in neurons of the dorsal horn and gracile nucleus after peripheral nerve injury

We have examined the mechanisms underlying Abeta-evoked c-fos expression in the dorsal horn and gracile nucleus following either sciatic nerve section or crush injury. The results indicate that in the spinal cord Abeta-evoked c-fos does not depend on primary afferent sprouting but is associated with the disconnection from the peripheral target since its expression in the dorsal horn reverts to normal after crush injury when regeneration occurs but persists after cut and ligation where regeneration is prevented. In contrast, however, Abeta-evoked c-fos expression in the gracile nucleus may be under some other control since its expression appears independent of peripheral nerve regeneration.

Influence of spinalization on spinal withdrawal reflex responses varies depending on the submodality of the test stimulus and the experimental pathophysiological condition in the rat

The influence of midthoracic spinalization on thermally and mechanically induced spinal withdrawal reflex responses was studied in the rat. There were three experimental groups of rats: healthy controls, rats with a spinal nerve ligation-induced unilateral neuropathy, and rats with a carrageenan-induced inflammation of one hindpaw. Tail flick response was induced by radiant heat. Hindlimb withdrawal was induced by radiant heat, ice water, and innocuous or noxious mechanical stimulation of the paw. Prior to spinalization, spinal nerve ligated and carrageenan-treated animals had a marked unilateral allodynia and hyperalgesia. Spinalization tended to induce a facilitation of noxious heat-evoked reflexes. This spinalization-induced facilitation was stronger on tail than hindlimb withdrawal. Spinalization-induced skin temperature change did not explain the facilitation of noxious heat-evoked reflexes. In contrast, spinal withdrawal responses induced by noxious cold or mechanical stimulation were significantly suppressed following spinalization. The spinalization-induced facilitatory effects as well as inhibitory ones on spinal reflexes were enhanced in inflamed/neuropathic animals. The results indicate that the tonic descending control of spinal nocifensive responses varies depending on the submodality of the test stimulus, the segmental level of the reflex (tail vs. hindlimb), and on the pathophysiological condition.

Somatotopic redistribution of c-fos expressing neurons in the superficial dorsal horn after peripheral nerve injury

The functional somatotopic reorganization of the lumbar spinal cord dorsal horn after nerve injury was studied in the rat by mapping the stimulus-evoked distribution of neurons expressing proto-oncogene c-fos. In three different nerve injury paradigms, the saphenous nerve was electrically stimulated at C-fibre strength at survival times ranging from 40 h to more than six months: 1) Saphenous nerve stimulation from three weeks onwards after ipsilateral sciatic nerve transection resulted in an increase in the number of Fos-immunoreactive neurons within the dorsal horn saphenous territory in laminae I-II, and an expansion of the saphenous territory into the denervated sciatic territory until 14 weeks postinjury. 2) Saphenous nerve stimulation from five days onwards after ipsilateral sciatic nerve section combined with saphenous nerve crush resulted in an increase in the number of Fos-immunoreactive neurons within the dorsal horn saphenous nerve territory, and an expansion of the saphenous nerve territory into the denervated sciatic nerve territory. 3) Stimulation of the crushed nerve (without previous adjacent nerve section) at five days, but not at eight months resulted in a temporary increase in the number of Fos-immunoreactive neurons within the territory of the injured nerve, and no change in area at either survival time. The results indicate that nerve injury results in an increased capacity of afferents in an adjacent uninjured, or regenerating nerve, to excite neurons both in its own and in the territory of the permanently injured nerve in the dorsal horn. The onset and duration of the increased postsynaptic excitability and expansion depends on the types of nerve injuries involved. These findings indicate the complexity of the central changes that follows in nerve injuries that contain a mixture of uninjured, regenerating and permanently destroyed afferents.

Inflammation-induced Fos protein expression in the rat spinal cord is enhanced following dorsolateral or ventrolateral funiculus lesions

Previous studies have shown an enhanced expression of Fos protein-like immunoreactivity in the lumbar spinal cord of rats with complete spinal transection following persistent hindpaw inflammation. To further locate the spinal pathways responsible for these effects, we compared the inflammation-evoked Fos expression in rats with bilateral lesions of the dorsolateral (DLFX) or ventrolateral (VLFX) funiculus, and with rats with a sham operation. The results indicate that the number of Fos-labeled neurons was significantly increased in all laminae of the dorsal horn ipsilateral to the inflamed hindpaw and in contralateral deep dorsal horn in both DLFX and VLFX rats compared to sham-operated rats. Moreover, when comparing DLFX and VLFX rats, in the ipsilateral spinal cord, DLFX resulted in more Fos expression in the deep dorsal horn; in contrast, a larger number of Fos-labeled cells in superficial laminae was observed in VLFX rats. These results suggest that modulatory systems, which descend in both DLF and VLF pathways, mediate the enhanced net descending nociceptive inhibition after persistent inflammation, although the supraspinal sites of origin of each pathway are likely functionally diverse.

Carrageenan-induced changes in spinal nociception and its modulation by the brain stem

Carrageenan was used to study inflammation-induced changes in spinal nociception and its brain stem modulation in the pentobarbitone-anesthetized rat. Carrageenan was administered intraplantarly into one hindpaw 2 h before the start of electrophysiological single unit recordings of wide-dynamic range (WDR) neurons of the spinal dorsal horn. Carrageenan produced a significant leftward shift in the stimulus-response function for mechanical stimuli, whereas that for noxious heat stimuli was short of statistical significance. Conditioning electrical stimulation in the rostroventromedial medulla (RVM) significantly attenuated noxious heat-evoked, but not mechanically evoked, responses to spinal dorsal horn WDR neurons in the control (contralateral) side. However, in the carrageenan-treated side RVM stimulation had no significant effect on mechanically or noxious heat-evoked responses. Following direct spinal administration of neuropeptide FF (NPFF), noxious heat-evoked responses, but not mechanically evoked responses, were attenuated by RVM-stimulation also in the carrageenan-treated side. This selective NPFF-induced enhancement of brain stem-spinal inhibition was not reversed by naloxone. The results indicate that carrageenan-induced inflammation significantly changes the response properties of spinal nociceptive neurons and their brain stem-spinal modulation. During inflammation, NPFF in the spinal cord produces a submodality-selective potentiation of the antinociceptive effect induced by brain stem-spinal pathways, independent of naloxone-sensitive opioid receptors.