c-fos antisense oligodeoxynucleotide increases formalin-induced nociception and regulates preprodynorphin expression

Effects of dimethyl itaconate on expressions of NGFI-A and NGFI-B and inflammatory cytokines in the spinal cord in the formalin test

Neural sensitization can cause neuroinflammation, which is a type of inflammation that occurs in both the peripheral nervous system and central nervous system. The purpose of this study was to investigate the effect of dimethyl itaconate (DMI) on the expression of NGFI-A and NGFI-B and inflammatory cytokines in the spinal cord in the formalin test. The rats were divided into five groups: control, formalin, DMI 10 mg/kg + formalin, DMI 20 mg/kg + formalin and diclofenac sodium 10 mg/kg + formalin. We evaluated the impact of DMI on the spinal cords NGFI-A and NGFI-B expressions and inflammatory and anti-inflammatory cytokines [interleukin-1 beta (IL-1β), tumour necrosis factor-alpha (TNF-α), interleukin-6 (IL-6) and interleukin-10 (IL-10)]. The findings indicate that DMI 10, DMI 20 and diclofenac sodium 10 mg/kg can relieve pain in rats during the formalin test. In addition, these substances were found to reduce the expression of NGFI-A and NGFI-B in the spinal cord. Moreover, DMI 10, DMI 20 and diclofenac sodium 10 mg/kg were observed to increase the expression of IL-10 while decreasing IL-1β, TNF-α and IL-6 in the spinal cord when compared with the formalin group. We have found that administering DMI can alleviate pain in rats during formalin test. Through our research, we have observed that DMI decreases the expression of NGFI-A and NGFI-B in the spinal cord. Furthermore, DMI has been shown to increase the levels of IL-10 while decreasing IL-1β, TNF-α and IL-6 in the spinal cord.

Antinociceptive effects of vitamin B-complex: A behavioral and histochemical study in rats

B-vitamins have been evaluated as a useful adjuvant therapy to treat pain. In spite of clinical and experimental evidence indicating the analgesic effect of B-vitamins, few studies have investigated their effect on aspects of the inflammatory pain response. In the present study, we investigated the analgesic effect of chronic application of B-complex vitamins (Neurobion) using an inflammatory experimental pain model in rats. Nociceptive behavioral responses were evaluated in male Wistar rats after plantar injection of formalin, comparing the treatment group (TG) with Neurobion pretreatment to the control group (CG) without the pretreatment. In addition, neuronal activity in the central pain pathway was evaluated using c-Fos immunohistochemical reactivity and NADPH-d histochemistry. A highly significant reduction of painful behaviors such as licking and flinching were observed in TG, especially during the secondary phase of the formalin test compared to CG. Results suggest that long-term pre-treatment using Neurobion can have a beneficial effect in reducing the chronic phase of pain. In addition, we observed a downregulation of c-Fos and NADPH-d in dorsal spinal neurons, suggesting that the antinociceptive effect induced by Neurobion could be due to a suppression of nociceptive transmission at the spinal level, particularly in the afferent regions of the dorsal spinal horn, which these neurons utilizing nitric oxide at least as one of their pain neurotransmitters.

Pulsed Radiofrequency of the Occipital Nerves: Results of a Standardized Protocol on Chronic Headache Management

BACKGROUND

Pulsed radiofrequency (PRF) of the occipital nerves has neuromodulative properties and is used for chronic pain management. However, its role in various types of chronic headaches has not been adequately investigated so far.

OBJECTIVES

Τhis was an observational, open-label, prospective study aiming to assess the efficacy of PRF of occipital nerves on various types of chronic headache management.

METHODS

Patients with chronic headaches followed up at the pain management unit were scheduled for PRF of both occipital nerves after a positive diagnostic nerve block. PRF was applied following a standardized protocol at 42°C, and the number of headaches per month was assessed as a primary outcome at baseline (before treatment), as well as after 1, 3, and 6 months. Pain intensity during headache crises was recorded using the Numeric Rating Scale (NRS, 0 - 10).

RESULTS

Fifty-seven patients suffering from chronic migraines, cluster headaches, tension-type headaches, and occipital neuralgia were studied. PRF significantly improved the number of headache episodes per month, as well as the pain intensity of the crises. The median number of headache episodes per month was significantly reduced in patients with migraine, from 14.5 to 4 after 1 month, and to 6.5 after 6 months. The same was seen for patients with clusters, who were also improved. A statistically significant reduction in NRS values over time was seen for all types of headaches.

CONCLUSIONS

PRF of the occipital nerves can lead to a reduction of the number of headache episodes per month, improving the intensity of pain during each episode.

Can Pulsed Radiofrequency of the Occipital Nerves Cause Sedation? A New Perspective of Existing Knowledge

BACKGROUND

Pulsed radiofrequency stimulation (PRF) of the greater and lesser occipital nerves (GON and LON) has neuromodulative and analgesic properties.

OBJECTIVES

The aim of this study was to investigate whether the PRF of the occipital nerves can cause sedation.

METHODS

This is an observational case series study in Pain Management Unit of Attikon University Hospital. Patients suffering from primary headaches were scheduled for PRF of occipital nerves following a standardized protocol. The Bispectral Index device was applied and recorded every minute throughout the procedure.

RESULTS

22 patients were studied. BIS values were lowered in all but three patients during GON stimulation, and in all patients during LON stimulation. Values of decline compared to baseline ranged between 0 - 23 (median 8.5) for GON and 1 - 27 (median 14) for LON, with LON decline being significantly lower.

CONCLUSIONS

PRF at the occipital nerves led to mild sedation in all patients. Further studies are required to investigate this effect and clarify the exact mode of action of pulsed radiofrequency.

D-limonene exhibits superior antihyperalgesic effects in a β-cyclodextrin-complexed form in chronic musculoskeletal pain reducing Fos protein expression on spinal cord in mice

Chronic musculoskeletal pain is one of the main symptoms found in Fibromyalgia with unclear etiology and limited pharmacological treatment. The aim of this study was to complex LIM in β-cyclodextrin (LIM-βCD) and then evaluate its antihyperalgesic effect in an animal model of chronic musculoskeletal pain. Differential scanning calorimetry and scanning electron microscopy was used for the characterization of the inclusion complex. Male Swiss mice were used for experimental procedures where mechanical hyperalgesia, thermal hyperalgesia, muscular strength, Fos immunofluorescence was studied after induction of hyperalgesia. Mechanism of action was also investigated through tail flick test and capsaicin-induced nociception. Endothermic events and morphological changes showed that the slurry complex method was the best method for the complexation. After induction of hyperalgesia, the oral administration of LIM-βCD (50mg/kg) significantly increased the paw withdrawal threshold compared to uncomplexed limonene. Fos immunofluorescence showed that both compounds significantly decreased the number of Fos-positive cells in the dorsal horn. In nociceptive tests, FLU was able to reverse the antinociceptive effect of LIM-βCD. After intraplantar administration of capsaicin, LIM was able to significantly decrease time to lick. LIM-βCD has antihyperalgesic action superior to its uncomplexed form, with possible action in the dorsal horn of the spinal cord. These results suggest the possible applicability of LIM, uncomplexed or complexed with βCD, in conditions such as FM and neuropathic pain, for which there are currently only limited pharmacological options.

Anti-hyperalgesic effect of Lippia grata leaf essential oil complexed with β-cyclodextrin in a chronic musculoskeletal pain animal model: Complemented with a molecular docking and antioxidant screening

BACKGROUND

Due to its unclear pathophysiology, the pharmacological treatment of fibromyalgia is a challenge for researchers. Studies using medicinal plants, such as those from the genus Lippia, complexed with cyclodextrins (CDs) have shown innovative results.

OBJECTIVE

The present research intended to evaluate the effect of an inclusion complex containing β-cyclodextrin (βCD) inclusion complex with Lippia grata (LG) essential oil in a chronic musculoskeletal pain model, its central activity and its possible interaction with neurotransmitters involved in pain.

METHODS

After acid saline-induced chronic muscle pain, male mice were evaluated for primary and secondary hyperalgesia and muscle strength. Moreover, an antagonist assay was performed to assess the possible involvement of the opioidergic, serotonergic and noradrenergic pathways. In addition, Fos protein in the spinal cord was assessed, and a docking study and antioxidant assays were performed.

RESULTS

The treatment with LG-βCD, especially in the dose of 24mg/kg, was able to significantly decrease (p<0.05) the paw withdrawal and muscle threshold. Furthermore, LG-βCD was shown to affect the opioidergic and serotonergic pathways. There were no significant changes in muscle strength. Fos protein immunofluorescence showed a significant decrease in expression in the dorsal horn of the spinal cord. The main compounds of LG showed through the docking study interaction energies with the alpha-adrenergic and μOpioid receptors. In all antioxidant assays, LG exhibited stronger antioxidant activities than LG-βCD.

CONCLUSION

This study suggested that LG-βCD could be considered as a valuable source for designing new drugs in the treatment of chronic pain, especially musculoskeletal pain.

REVIEW ■ : Jun, Fos, and CREB/ATF Transcription Factors in the Brain: Control of Gene Expression under Normal and Pathophysiological Conditions

The expression and activation of transcription factors and the control of gene transcription in the nervous system is a recent and rapidly expanding field in neurosciences. This research area may provide insights concerning the information transfer that arises from postsynaptic potentials or ligand-coupling of membrane receptors and terminates in gene expression. Visualization of both de novo synthesis of inducible transcription factors (ITFs) and phosphorylation of preexisting transcription factors have been used to mark neurons, pathways, and networks excited by various stimuli. This article summarizes basics of the transcription process and the complex functions of Jun, Fos, and CREB/ATF proteins, as well as the use of ITFs as experimental instruments in neurophysiology and neurobiology. The major focus is on the alterations in ITF expression following acute or chronic pathophysiological stimuli as mirrors of alterations in neuronal programs underlying adaptation, dysfunctions, or the development of diseases affecting the nervous system. NEUROSCIENTIST 2:153-161, 1996

Pulsed radiofrequency treatment of articular branches of femoral and obturator nerves for chronic hip pain

Purpose

Chronic hip pain is a common symptom experienced by many people. Often, surgery is not an option for patients with multiple comorbidities, and conventional drugs either have many side effects or are ineffective. Pulsed radiofrequency (PRF) is a new method in the treatment of pain. We attempt to compare the efficacy of PRF relative to conservative management for chronic hip pain.

RPatients and methods

Between August 2011 and July 2013, 29 patients with chronic hip pain were divided into two groups (PRF and conservative treatment) according to consent or refusal to undergo PRF procedure. Fifteen patients received PRF of the articular branches of the femoral and obturator nerves, and 14 patients received conservative treatment. Visual analog scale (VAS), Oxford hip scores (OHS), and pain medications were used for outcome measurement before treatment and at 1 week, 4 weeks, and 12 weeks after treatment.

Results

At 1 week, 4 weeks, and 12 weeks after treatment initiation, improvements in VAS were significantly greater with PRF. Improvements in OHS were significantly greater in the PRF group at 1 week, 4 weeks, and 12 weeks. Patients in the PRF group also used less pain medications. Eight subjects in the conservative treatment group switched to the PRF group after 12 weeks, and six of them had >50% improvement.

Conclusion

When compared with conservative treatment, PRF of the articular branches of the femoral and obturator nerves offers greater pain relief for chronic hip pain and can augment physical functioning.

Antinociceptive Effects of Heterotopic Electroacupuncture in Formalin-Induced Pain

This study examined the antinociceptive effect of electroacupuncture (EA) to heterotopic acupoints on formalin-induced pain in rats. EA (2 ms, 10 Hz, and 3 mA) was delivered to heterotopic acupoints HE7 and PE7, or non-acupoints at the right fore limb, for 30 min and was immediately followed by subcutaneous formalin injection into the left hind paw, respectively. The quantified pain score, electromyogram (EMG) response of the C-fiber reflex, and cFos immunoreactivity were assessed, respectively. EA to heterotopic acupoints significantly reduced both early- and late-phase pain-like behaviors and significantly decreased the EMG responses of the C-fiber reflex after formalin injection. By contrast, EA to non-acupoints had no significant effects on pain-like behavior or the EMG response. In addition, EA to heterotopic acupoints decreased cFos immunoreactivity in the lumbar spinal dorsal horn. Therefore, EA induced pre-emptive antinociception via the extra-segmental inhibition of the formalin-induced pain, suggesting that EA to heterotopic acupoints is a useful treatment for inflammatory pain.

Treatment experience of pulsed radiofrequency under ultrasound guided to the trapezius muscle at myofascial pain syndrome -a case report-

Trigger point injection treatment is an effective and widely applied treatment for myofascial pain syndrome. The trapezius muscle frequently causes myofascial pain in neck area. We herein report a case in which direct pulsed radiofrequency (RF) treatment was applied to the trapezius muscle. We observed that the RF treatment produced continuous pain relief when the effective duration of trigger point injection was temporary in myofascial pain.

Involvement of microglial P2X7 receptors and downstream signaling pathways in long-term potentiation of spinal nociceptive responses

Tetanic stimulation of the sciatic nerve (TSS) produces long-term potentiation (LTP) of C-fiber-evoked field potentials in the spinal cord. This potentiation is considered to be a substrate for long-lasting sensitization in the spinal pain pathway. Because microglia have previously been shown to regulate the induction of spinal LTP, we hypothesize that P2X7 receptors (P2X7R), which are predominantly expressed in microglia and participate in the communication between microglia and neurons, may play a role in this induction. This study investigated the potential roles of P2X7Rs in spinal LTP and persistent pain induced by TSS in rats. OxATP or BBG, a P2X7R antagonist, prevented the induction of spinal LTP both in vivo and in spinal cord slices in vitro and alleviated mechanical allodynia. Down-regulation of P2X7Rs with P2X7-siRNA blocked the induction of spinal LTP and inhibited mechanical allodynia. Double immunofluorescence showed colocalization of P2X7Rs with the microglial marker OX-42, but not with the astrocytic marker GFAP or the neuronal marker NeuN. Intrathecal injection of BBG suppressed the up-regulation of microglial P2X7Rs and increased expression of Fos in the spinal superficial dorsal horn. Further, pre-administration of BBG inhibited increased expression of the microglial marker Iba-1, phosphorylated p38 (p-p38), interleukin 1β (IL-1β) and GluR1 following TSS. Pre-administration of the IL-1 receptor antagonist (IL-1ra) blocked both the induction of spinal LTP and the up-regulation of GluR1. These results suggest that microglial P2X7Rs and its downstream signaling pathways play a pivotal role in the induction of spinal LTP and persistent pain induced by TSS.

Increased spinal dynorphin levels and phospho-extracellular signal-regulated kinases 1 and 2 and c-Fos immunoreactivity after surgery under remifentanil anesthesia in mice

In humans, remifentanil anesthesia enhances nociceptive sensitization in the postoperative period. We hypothesized that activation of extracellular signal-regulated kinases 1 and 2 (ERK1/2) and the expression of c-Fos, prodynorphin (mRNA), and dynorphin in the spinal cord could participate in the molecular mechanisms underlying postoperative opioid-induced sensitization. In a mouse model of incisional pain, we evaluated thermal (Hargreaves test) and mechanical (von Frey) hyperalgesia during the first 21 postoperative days. Moreover, prodynorphin (mRNA, real-time polymerase chain reaction), dynorphin (enzymatic immunoassay), c-Fos expression, and ERK1/2 phosphorylation (both by immunohistochemistry) in the lumbar spinal cord were assessed. Surgery performed under remifentanil anesthesia induced a maximal decrease in nociceptive thresholds between 4 h and 2 days postoperatively (p < 0.001) that lasted 10 to 14 days compared with noninjured animals. In the same experimental conditions, a significant increase in prodynorphin mRNA expression (at 2 and 4 days) followed by a sustained increase of dynorphin (days 2 to 10) in the spinal cord was observed. We also identified an early expression of c-Fos immunoreactivity in the superficial laminae of the dorsal horn of the spinal cord (peak at 4 h; p < 0.001), together with a partial activation of ERK1/2 (4 h; p < 0.001). These findings suggest that activated ERK1/2 could induce c-Fos expression and trigger the transcription of prodynorphin in the spinal cord. This in turn would result in long-lasting increased levels of dynorphin that, in our model, could participate in the persistence of pain but not in the manifestation of first pain.

A case series of pulsed radiofrequency treatment of myofascial trigger points and scar neuromas

INTRODUCTION

Pulsed radiofrequency (PRF) current applied to nerve tissue to treat intractable pain has recently been proposed as a less neurodestructive alternative to continuous radiofrequency lesioning. Clinical reports using PRF have shown promise in the treatment of a variety of focal, neuropathic conditions. To date, scant data exist on the use of PRF to treat myofascial and neuromatous pain.

METHODS

All cases in which PRF was used to treat myofascial (trigger point) and neuromatous pain within our practice were evaluated retrospectively for technique, efficacy, and complications. Trigger points were defined as localized, extremely tender areas in skeletal muscle that contained palpable, taut bands of muscle.

RESULTS

Nine patients were treated over an 18-month period. All patients had longstanding myofascial or neuromatous pain that was refractory to previous medical management, physical therapy, and trigger point injections. Eight out of nine patients experienced 75-100% reduction in their pain following PRF treatment at initial evaluation 4 weeks following treatment. Six out of nine (67%) patients experienced 6 months to greater than 1 year of pain relief. One patient experienced no better relief in terms of degree of pain reduction or duration of benefit when compared with previous trigger point injections. No complications were noted.

DISCUSSION

Our review suggests that PRF could be a minimally invasive, less neurodestructive treatment modality for these painful conditions and that further systematic evaluation of this treatment approach is warranted.

c-Fos and pERK, which is a better marker for neuronal activation and central sensitization after noxious stimulation and tissue injury?

c-Fos, the protein of the protooncogene c-fos, has been extensively used as a marker for the activation of nociceptive neurons in the spinal cord for more than twenty years since Hunt et al. first reported that peripheral noxious stimulation to a hind paw of rats leads to a marked induction of c-Fos in superficial and deep dorsal horn neurons in 1987. In 1999, Ji et al. reported that phosphorylated extracellular signal-regulated kinase (pERK) is specifically induced by noxious stimulation in superficial dorsal horn neurons. Accumulating evidence indicates that pERK induction or ERK activation in dorsal horn neurons is essential for the development of central sensitization, increased sensitivity of dorsal horn neurons that is responsible for the generation of persistent pain. Further, molecular mechanisms underlying ERK-mediated central sensitization have been revealed. In contrast, direct evidence for c-Fos-mediated central sensitization is not sufficient. After a noxious stimulus (e.g., capsaicin injection) or tissue injury, c-Fos begins to be induced after 30-60 minutes, whereas pERK can be induced within a minute, which can correlate well with the development of pain hypersensitivity. While c-Fos is often induced in the nuclei of neurons, pERK can be induced in different subcellular structures of neurons such as nuclei, cytoplasma, axons, and dendrites. pERK can even be induced in spinal cord microglia and astrocytes after nerve injury. In summary, both c-Fos and pERK can be used as markers for neuronal activation following noxious stimulation and tissue injury, but pERK is much more dynamic and appears to be a better marker for central sensitization.

Spatiotemporal characteristics of pain-associated neuronal activities in primary somatosensory cortex induced by peripheral persistent nociception

The primary somatosensory cortex (S1 area) is one of the key brain structures for central processing of somatic noxious information to produce pain perception. However, so far, the spatiotemporal characteristics of neuronal activities associated with peripheral persistent nociception have rarely been studied. In the present report, we used c-Fos as a neuronal marker to analyze spatial and temporal patterns of pain-related neuronal activities within the S1 area of rats subjecting to subcutaneous (s.c.) injection of bee venom (BV) solution, a well-established animal model of persistent pain. In naïve and saline-treated rats, c-Fos-labeled neurons were diffusely and sparsely distributed in the hindlimb region of S1 area. Following s.c. BV injection, c-Fos-labeled neurons became densely increased in superficial layers (II-III) and less increased in deep layers (IV-VI). The mean number of c-Fos positive neurons in the layers II-III began to increase at 1h and reached a peak at 2h after BV treatment that was followed by a gradual decrease afterward. The time course of c-Fos expression in the layers IV-VI was in parallel with that of the superficial layers, but with a much lower density and magnitude. The present results demonstrated that BV-induced peripheral persistent nociception could evoke increased neuronal activities in the S1 area with predominant localization in layers II-III.

Percutaneous pulsed radiofrequency in the treatment of cervical and lumbar radicular pain

BACKGROUND

Previous reports of the efficacy of percutaneous pulsed radiofrequency have been confounded by fewer case numbers, poor patient selection, and limited data on cervical or lumbar radicular pain. We used percutaneous pulsed radiofrequency for cervical and lumbar radicular pain, and the study has more than 100 cases for the analysis of the efficacy of percutaneous pulsed radiofrequency.

METHODS

We collected 154 cases of patients with lumbar or cervical radicular pain due to a herniated intervertebral disk or previous failed surgery. They underwent pulsed radiofrequency therapy in 2 to 4 spinal levels unilaterally. Follow-up period was from 1 week to 1 year postoperatively.

RESULTS

Twenty-six (53.06%) of 49 patients and 59 (50.86%) of 116 patients after cervical and lumbar pulsed radiofrequency stimulation, respectively, had an initial improvement of 50% or more in the first week of follow-up. Twenty-seven (55.10%) of 49 patients and 52 (44.83%) of 116 patients after cervical and lumbar pulsed radiofrequency stimulation, respectively, had pain relief of 50% or more at the follow-up period of 3 months. In the analysis of patients with pain relief of 50% or more for at least 1 month, the most effective period was during postoperation 1 month later. No complication was found among these patients.

CONCLUSIONS

The results of this retrospective analysis showed that the application of pulsed radiofrequency is a safe and useful intervention for cervical and lumbar radicular pain. The satisfactory pain relief obtained by most of our patients justifies the start of this study for at least 6 months. Although pulsed radiofrequency appears to provide intermediate-term relief of pain, further studies with long-term follow-up are necessary.

Prolonged nociceptive responses to hind paw formalin injection in rats with a spinal cord injury

Unilateral lesioning of the spinal dorsal horn with the excitotoxin quisqualic acid (QUIS) leads to robust degeneration of dorsal horn grey matter, and robust pain-related symptoms, such as cutaneous hypersensitivity, persist long after injury. A possible mechanism that underlies the pain-related symptoms is the disruption of dorsal horn inhibitory neuron function, leading to decreased inhibition of nociceptive neurons. Five percent formalin was injected into the hind paw of rats with either a QUIS lesion or sham lesion. Both QUIS- and sham-lesioned rats displayed bi-phasic hind paw flinches following formalin injection, but a prolonged response was observed in QUIS-lesioned rats. The expression of the immediate-early gene product Fos in the dorsal horn ipsilateral to formalin injection was similar between QUIS- and sham-lesioned rats. In QUIS-lesioned rats, however, there was a marked absence of dorsal horn neurons, particularly GABAergic neurons, compared to sham-lesioned rats. The prolonged nociceptive response observed with a unilateral QUIS lesion may be due to generalized changes in dorsal horn neuron function including a loss of inhibitory neuron function.

The involvement of spinal bovine adrenal medulla 22-like peptide, the proenkephalin derivative, in modulation of nociceptive processing

Bovine adrenal medulla 22 (BAM22), one of the cleavage products of proenkephalin A, possesses high affinity for opioid receptors and sensory neuron-specific receptor (SNSR). The present study was designed to examine the expression of BAM22 in the spinal cord and dorsal root ganglion (DRG) of naive rats as well as in a model of inflammation. BAM22-like immunoreactivity (BAM22-IR) was expressed in fibers in the spinal cord, with high density seen in lamina I in naïve rats. The expression of BAM22-IR in the superficial laminae was greatly reduced following dorsal rhizotomy. BAM22-IR was also located in 19% of DRG cells, mainly in the small- and medium-sized subpopulations. Following injection of complete Freund's adjuvant (CFA) in the hindpaw, the expression of BAM22-IR in the superficial laminae of the spinal cord and small-sized DRG neurons on the ipsilateral side was markedly increased. Double labeling showed that the Fos-positive nucleus was surrounded by BAM22-IR cytoplasm in the spinal dorsal horn neurons or closely associated with BAM22-IR fibers in the superficial laminae. Furthermore, CFA-induced mechanical allodynia in the inflamed paw was potentiated by intrathecal administration of anti-BAM22 antibody. Together, these results demonstrate for the first time that BAM22-like peptide is mainly located in the superficial laminae of the spinal cord and mostly originates from nociceptive DRG neurons. BAM22 could thus act as a ligand for presynaptic opioid receptors and SNSR. Our study also provides evidence suggesting that BAM22 plays a role in the modulation of nociceptive processing at the spinal level under normal and inflammatory conditions.

Neuronal activation at the spinal cord and medullary pain control centers after joint stimulation: a c-fos study in acute and chronic articular inflammation

Chronic inflammatory pain induces short- and long-term central changes, which have been mainly studied at the spinal cord level. Supraspinal pain control centers intrinsically connected with the dorsal horn are also prone to be affected by chronic inflammatory pain. C-fos expression was used as a neuronal activation marker at spinal and supraspinal levels to i) compare acute and chronic articular inflammation, and ii) analyze the effects of brief innocuous or noxious stimulation of a chronically inflamed joint. Acute articular inflammation was induced by an inflammatory soup with prostaglandin E(2) and bradykinin, both at 10(-5) M. Chronic articular inflammation consisted of 14 days of monoarthritis. Early c-fos expression was studied 4 min after inflammatory soup injection or stimulation of the arthritic joint whereas late c-fos expression was evaluated 2 h after those stimuli. At the spinal cord, the analysis was focused on the dorsal horn (laminae I-V) and supraspinally, five major regions of the endogenous pain control system were considered: the caudal ventrolateral medulla (VLM), the dorsal reticular nucleus (DRt), the ventral reticular nucleus (VRt), the nucleus of the solitary tract (Sol) and the rostroventromedial medulla (RVM). Acute articular inflammation induced early and late increases in c-fos expression at the spinal level and late increases supraspinally whereas the effects of monoarthritis were more moderate and restricted to the spinal cord. When monoarthritic animals were subjected to gentle touch or bending of the joint, early increases in c-fos expression were detected supraspinally, but not at the spinal level. In this region, noxious mechanical stimulation induced late increases in non-inflamed animals and both early and late increases in monoarthritic rats. Supraspinally, noxious stimulation induced only late increases in c-fos expression. The present results show complex differences in the patterns of c-fos expression between the spinal cord and medullary areas of the pain control system during articular inflammation, which indicate that the somatosensory system is differentially affected by the installation of chronic pain.

Expansion of formalin-evoked Fos-immunoreactivity in rats with a spinal cord injury

Peripheral tissue injury as well as spinal cord injury (SCI) may lead to sensitization of dorsal horn neurons and alterations in nociceptive processing. Thus, peripheral injuries experienced by SCI patients, even if not initially perceived, could result in a persistent and widespread activation of dorsal horn neurons and emerge as chronic pain with interventive repair or modest recovery from SCI. To visualize the spinal neuron response to peripheral tissue injury following complete SCI in rats, the neural transcription factor Fos was quantitated in the spinal cord. Two weeks following either a complete transection of the spinal cord at the level of T8 or a sham surgery (laminectomy), rats were injected with formalin into the left hind paw. Sham-operated rats demonstrated biphasic hind paw pain-related behavior following formalin injection, but transected rats displayed fewer behaviors in the second (tonic) phase. Stereological analysis of the sham group revealed that the extent of formalin-induced Fos expression was within the lumbar dorsal horn, with numerous Fos-like immunoreactive profiles in the ipsilateral dorsal horn and some contralateral immunoreactive profiles. In contrast, the level of Fos-like immunoreactivity in the transected group was significantly elevated and expanded in range compared to the sham group, with increases observed in the normal laminar distribution regions, as well as multi-segmentally through sacral levels and increases in the contralateral dorsal horn segments. The data demonstrate that widespread activation of spinal, especially dorsal horn, neurons following peripheral insult can occur in the injured spinal cord, despite reduced pain responsiveness, and suggests that exaggerated pain may emerge as spinal recovery or repair progresses.

Differential activation of protein kinases in the dorsal horn in vitro of normal and inflamed rats by group I metabotropic glutamate receptor subtypes

Group I metabotropic glutamate receptors (mGluRs) contribute to spinal sensitization and synaptic plasticity but the underlying mechanisms are unknown. Here, group I mGluR modulation of evoked monosynaptic excitatory postsynaptic currents (EPSCs) in substantia gelatinosa (SG) neurones in vitro was investigated in juvenile rats. In addition, the role of group I mGluRs in dorsal horn neuronal Fos expression was determined in tetrodotoxin (TTX)-treated in vitro spinal cords of naïve rats and those with Complete Freund's Adjuvant (CFA) peripheral inflammation. In the majority of SG neurones, (S)-3,5-dihydroxyphenylglycine (DHPG) reduced EPSCs and this effect was inhibited by the mGluR(5) antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP). Data for paired-pulse and spontaneous miniature excitatory postsynaptic currents (mEPSCs) suggest mGluR(5) acts presynaptically to reduce transmitter release. DHPG-induced reduction of EPSC amplitude operated via PKC, but not ERK, signalling cascade. In the dorsal horn of naïve but not CFA rats, DHPG increased Fos expression and this was reduced by MPEP and both PKC and ERK inhibitors. In the CFA group, basal Fos expression was reduced by MPEP and the kinase inhibitors. These data infer a role for mGluR(5) in acute modulation of nociceptive synaptic efficacy within the dorsal horn and postsynaptic activation of transcription factors such as Fos that are implicated in activity-dependent neuroplastic adaptation. These actions are achieved by differential activation of PKC- and ERK-dependent transduction pathways.

Mechanisms of orofacial pain control in the central nervous system

Recent advances in the study of pain have revealed somatotopic- and modality-dependent processing and the integration of nociceptive signals in the brain and spinal cord. This review summarizes the uniqueness of the trigeminal sensory nucleus (TSN) in structure and function as it relates to orofacial pain control. The oral nociceptive signal is primarily processed in the rostral TSN above the obex, the nucleus principalis (Vp), and the subnuclei oralis (SpVo) and interpolaris (SpVi), while secondarily processed in the subnucleus caudalis (SpVc). In contrast, the facial nociceptive signal is primarily processed in the SpVc. The neurons projecting to the thalamus are localized mostly in the Vp, moderately in the SpVi, and modestly in the ventrolateral SpVo and the SpVc. Orofacial sensory inputs are modulated in many different ways: by interneurons in the TSN proper, through reciprocal connection between the TSN and rostral ventromedial medulla, and by the cerebral cortex. A wide variety of neuroactive substances, including substance P, gamma-aminobutyric acid, serotonin and nitric oxide (NO) could be involved in the modulatory functions of these curcuits. The earliest expression of NO synthase (NOS) in the developing rat brain is observed in a discrete neuronal population in the SpVo at embryonic day 15. NOS expression in the SpVc is late at postnatal day 10. The neurons receiving intraoral signals are intimately related with the sensorimotor reflexive function through the SpVo. In summary, a better understanding of the trigeminal sensory system--which differs from the spinal system--will help to find potential therapeutic targets and lend to developing new analgesics for orofacial-specific pain with high efficacy and fewer side effects.

c-Fos antisense oligodeoxynucleotide offsets behavioral nociceptive responses and both up-regulations of c-Fos protein and dynorphin a (1-8) in dorsal horn: a study using the formalin test in rats

The formalin test was used to elicit acute and chronic pain in rats, and antisense oligodeoxynucleotide (AS-ODN) was used as a tool to modulate the expression of nociceptive behavioral and neurochemical responses. AS-ODN complementary to c-Fos mRNA was administered intrathecally (i.t.) 4 h before formalin injection in the experimental group. Normal saline or reverse AS-ODN was pre-administered i.t. at the same time in two control groups (saline and reverse AS-ODN). The results showed that the acute phase of nociceptive behavior showed no change by AS-ODN administration, whereas the tonic phase of nociceptive licking and biting behavior was significantly suppressed by AS-ODN as compared with the saline or the reverse AS-ODN group, respectively (p < .05 and p < .01). At the same time, both Fos-like immunoreactive (FLI) neurons and density of dynorphin-like immunoreactivities (DLI) were decreased significantly (p < .05 and p < .01) in the AS-ODN group as compared with that in two control groups. The results indicate that the long-lasting nociceptive responses elicited by sustained noxious inputs are based on the up-regulation of c-Fos gene expression, which in turn induces the upregulation of Dyn A production. It is proposed that intensified Dyn A production in the dorsal horn may be pivotal for the appearance of chronic pain.

Spinal distribution and metabolism of 2'-O-(2-methoxyethyl)-modified oligonucleotides after intrathecal administration in rats

Intrathecal (IT) delivery of antisense oligodeoxynucleotides (ASO) has been used to study the function of specific gene products in spinal nociception. However, a lack of systematic studies on the spinal distribution and kinetics of IT ASO is a major hurdle to the utilization of this technique. In the present study, we injected rats IT with 2'-O-(2-methoxyethyl) modified phosphorothioate ASO (2'-O-MOE ASO) and examined anatomical and cellular location of the ASO in the spinal cord and dorsal root ganglia (DRG) by immunocytochemistry. At 0.5 h after a single IT injection, immunostaining for ISIS 13920 (a 2'-O-MOE ASO targeting h-ras) localized superficially in the lumbar spinal cord, while at 24 h the immunostaining was distributed throughout the spinal cord and was predominantly intracellular. Double staining with cell type specific antibodies indicated that the ASO was taken up by both glia and neurons. ASO immunoreactivity was also observed in DRG after IT ISIS 13920. Capillary gel electrophoresis analysis showed that ISIS 22703, a 2'-O-MOE ASO targeting the alpha isozyme of protein kinase C (PKC), remained intact in spinal cord tissue and cerebrospinal fluid up to 24 h after the injection and no metabolites were detected. In contrast, after IT ISIS 11300, an unmodified phosphorothioate ASO with the same sequence as ISIS 22703, no full-length compound was detectable at 24 h, and metabolites were seen as early as 0.5 h. IT treatment with ISIS 22703 at doses that effectively down-regulated PKCalpha mRNA in spinal cord did not affect the mRNA expression in DRG. In summary, 2'-O-MOE ASO displayed high stability in spinal tissue after IT delivery, efficiently distributed to spinal cord, and internalized into both neuronal and non-neuronal cells. ASO are able to reach DRG after IT delivery; however, higher doses may be required to reduce target gene in DRG as compared with spinal cord.

Effects of intrathecal BAM22 on noxious stimulus-evoked c-fos expression in the rat spinal dorsal horn

The effects of bovine adrenal medulla 22 (BAM22), a cleaved product of proenkephalin A, were investigated on the noxious stimulus-evoked expressions of spinal c-fos-like immunoreactivity (FLI). Heat (51 degrees C) applied to the tail evoked FLI predominantly in laminae I-II of the sacral spinal cord. Intrathecal (i.t.) BAM22 at a dose of 7 nmol decreased the expressions of the heat-evoked FLI by 68%, 64% and 56% in laminae I-II, III-IV and V-VI, respectively, and the decrease pattern was comparable to that induced by i.t. morphine (10 mug). Naloxone (1 mg/kg, i.p.) significantly enhanced the heat-evoked FLI in laminae III-VI, prevented the morphine-induced inhibition, and decreased the potencies of BAM22 in laminae I-II and V-VI by 23-40%. Higher dose of naloxone (10 mg/kg, i.p.) also partially reduced the BAM22-induced suppression. Following intraplantar injection of formalin (2.5%), FLI neurons were preferentially distributed not only in laminae I-II but also in laminae III-IV and V-VI of segments L4-L5. Pretreatment with BAM22 (7 nmol, i.t.) reduced the formalin-evoked FLI neurons by 72%, 61% and 58%, in laminae I-II, III-IV and V-VI, respectively. Naloxone (1 mg/kg. i.p.) enhanced the formalin-evoked expressions of FLI in laminae III-VI and decreased the potencies of BAM22 by 22-38% in laminae I-II and V-VI. The present study provided evidence at a cellular level showing that opioid and non-opioid effects of BAM22 on nociceptive processing in acute and persistent pain models were associated with modulation of noxious stimulus-evoked activity of the spinal dorsal horn neurons.

Alterations in spinal cord gene expression after hindpaw formalin injection

Heme oxygenase type 2 (HO-2) is an enzyme that uses heme as a substrate to produce iron, biliverdin, and carbon monoxide (CO). This enzyme participates in regulation of nociceptive signal transmission in spinal cord tissue. We set out to identify genes undergoing alterations in expression in a model of inflammatory pain and to determine whether HO-2 participates in that regulation. After the hindpaw injection of formalin in mice, we measured changes in expression of immediate early genes including c-fos, c-jun, jun B, nerve growth factor induced genes (NGFI-A and NGFI-B) and activity-related cytoskeletal protein (ARC) using real-time PCR. The mRNA corresponding to these genes increased in abundance in the first hour after formalin injection and then slowly declined. Changes in the abundance of prodynorphin, extracellular signal related kinases (ERK1 and ERK2) and N-methyl-D-aspartate (NMDA) receptor R1 subunit mRNA generally peaked between 8 and 12 hr after formalin injection. In HO-2 null mutant mice, the enhancement of expression was less for all genes studied. We went on to quantify gene expression in superficial dorsal horn tissue using laser capture microdissection followed by RNA amplification and real-time PCR. The results confirmed that the changes in gene expression were occurring in regions of the spinal cord involved in nociceptive processing. We conclude that the hindpaw injection of formalin leads to enhanced early and late expression of many genes in spinal cord dorsal horn tissue, and that this enhancement of expression relies to a degree on the presence of HO-2.

Endogenous and exogenous glucocorticoid suppresses up-regulation of preprodynorphin mRNA and hyperalgesia in rats with peripheral inflammation

Glucocorticoids (GC) play important roles in response to stressful stimuli, including pain. This study examined the effects of bilateral adrenalectomy (ADX) and dexamethasone (DEX) replacement on the hyperalgesia and spinal preprodynorphin (PPD) mRNA expression induced by injecting complete Freund's adjuvant (CFA) into rats' hind paws. The results demonstrated that CFA induced more intense hyperalgesia and up-expression of spinal PPD mRNA in ADX rats than in control rats, while both of these intensified reactions could be significantly suppressed by subcutaneous pretreatment with DEX. This leads to the conclusion that both exogenous (pharmacological) and endogenous (physiological) GC suppresses the behavioral hyperalgesia and the up-regulation of spinal PPD mRNA induced by sustained peripheral inflammation. The results also suggest that spinal PPD mRNA suppression may partially underlie the inhibition of behavioral hyperalgesia.

Allodynia and hyperalgesia produced by specific inhibition of spinal c-fos expression: lack of correlation with dynorphin content

Inhibition of spinal Fos expression increases formalin-induced nociception and decreases spinal prodynorphin messenger ribonucleic acid (mRNA), suggesting that Fos modulates nociception by inducing dynorphin synthesis. This study tests the hypothesis that Fos modulates sensitivity to other somatic stimuli, such that inhibition of Fos expression will result in tactile allodynia and thermal hyperalgesia. In addition, it correlates the somatosensory effects of inhibition of Fos expression with spinal dynorphin content. Antisense oligodeoxynucleotide (ODN) to c-fos mRNA was administered by intrathecal infusion. Tactile sensitivity was tested by probing the hindpaw with von Frey filaments. Thermal sensitivity was quantitated by using withdrawal latency to radiant heat. Two percent formalin was injected into the dorsal hindpaw, and flinches were quantitated. Fos was quantitated by counting immunoreactive cells. Dynorphin was measured by immunoassay. Intrathecal antisense, but not mismatch, ODN resulted in tactile allodynia, thermal hyperalgesia, and hyperalgesia to formalin-induced nociception. Antisense ODN decreased Fos-like immunoreactivity after formalin injection but did not alter Jun-like immunoreactivity. Antisense ODN had differing effects on spinal dynorphin content, depending on the method of administration. These experiments show a role of Fos in modulating somatosensory sensitivity and suggest that induction of dynorphin synthesis is not the sole mechanism by which Fos does so.

Enhanced thermal antinociceptive potency and anti-allodynic effects of morphine following spinal administration of endotoxin

Recently, an animal model of central inflammation characterized by widespread cutaneous hyperalgesia and allodynia following intracerebroventricular (i.c.v.) administration of lipopolysaccharide (LPS) was described. In the present study, we demonstrate that central administration of LPS via intrathecal (i.t.) injection produces bilateral tactile allodynia and thermal hyperalgesia in the rat. Also, the effects of morphine-induced antinociception were determined in this model. Here we demonstrate enhanced thermal antinociceptive potency of i.t. morphine in LPS-treated rats compared to controls. Intrathecal morphine was also effective in alleviating the tactile allodynia induced by LPS. Both the antinociceptive and anti-allodynic effects produced by i.t. morphine were completely antagonized by pretreatment with subcutaneous naloxone (1 mg x kg(-1)). This study demonstrates the presence of both heat hyperalgesia and mechanical allodynia following central administration of LPS, and an increased antinociceptive potency of i.t. morphine in this model.

c-Fos expression in rat spinal cord induced by scorpion BmK venom via plantar subcutaneous injection

The aim of this study was to assess the cell-type and distribution of highly activated neurons in rat spinal cord underlying nociceptive responses induced by scorpion BmK venom using Fos immunohistochemistry. BmK venom was intraplantarly injected into one hind paw of a conscious rat. Fos-like immunoreactive neurons were found to predominantly distribute at L4-5 segments in the rat spinal cord after BmK venom application. c-Fos labeling was most dense in the medial half portion of laminae I-II, moderately dense in laminae V-VI and less dense in laminae III-IV, VII-X. c-Fos labeling could be detected at 0.5 h, reached the peak at 2 h, decreased steeply from 4 h and then almost disappeared at 24 h. Ten to fifty micrograms of BmK venom was deemed to be a sufficient dosage to evoke c-Fos expression. On the other hand, c-Fos expression induced by BmK venom could be suppressed partially by systemic morphine in a dose-dependent manner. The results suggest that the different extent of activities of neuronal subpopulation in the spinal cord involved in nociceptive transmission manifesting as c-Fos expression, were mainly correlated with mechanisms underlying the generation, maintenance and/or modulation of spontaneous pain and hyperalgesia evoked by BmK venom.

Role of NO in modulating neuronal activity in superficial dorsal horn of spinal cord during exercise pressor reflex

Static contraction of hindlimb skeletal muscle in cats induces a reflex pressor response. The superficial dorsal horn of the spinal cord is the major site of the first synapse of this reflex. In this study, static contraction of the triceps surae muscle was evoked by electrical stimulation of the tibial nerve for 2 min in anesthetized cats (stimulus parameters: two times motor threshold at 30 Hz, 0.025-ms duration). Ten stimulations were performed and 1-min rest was allowed between stimulations. Muscle contraction caused a maximal increase of 32 +/- 5 mmHg in mean arterial pressure (MAP), which was obtained from the first three contractions. Activated neurons in the superficial dorsal horn were identified by c-Fos protein. Distinct c-Fos expression was present in the L6-S1 level of the superficial dorsal horn ipsilateral to the contracting leg (88 +/- 14 labeled cells per section at L7), whereas only scattered c-Fos expression was observed in the contralateral superficial dorsal horn (9 +/- 2 labeled cells per section, P < 0.05 compared with ipsilateral section). A few c-Fos-labeled cells were found in control animals (12 +/- 5 labeled cells per section, P < 0.05 compared with stimulated cats). Furthermore, double-labeling methods demonstrated that c-Fos protein coexisted with nitric oxide (NO) synthase (NOS) positive staining in the superficial dorsal horn. Finally, an intrathecal injection of an inhibitor of NOS, N-nitro-L-arginine methyl ester (5 mM), resulted in fewer c-Fos-labeled cells (58 +/- 12 labeled cells per section) and a reduced maximal MAP response (20 +/- 3 mmHg, P < 0.05). These results suggest that the exercise pressor reflex induced by static contraction is mediated by activation of neurons in the superficial dorsal horn and that formation of NO in this region is involved in modulating the activated neurons and the pressor response to contraction.

Inhibitory effects of intrathecally administered interleukin-1beta on carrageenan-induced hyperalgesia and spinal c-Fos expression in rats

The present study investigated the effects of interleukin-1beta (IL-1beta) injected intrathecally (i.t.) on carrageenan-induced thermal hyperalgesia and spinal c-Fos expression. The paw withdrawal latency (PWL) by the thermal stimulus was taken as an index of the thermal hyperalgesia of rats. I.t. injection of 10 ng IL-1beta significantly increased the PWL of the carrageenan-injected paw. Expression of c-Fos induced by intraplantar (i.pl.) injection of carrageenan was examined in the spinal cord with immunohistochemical methods. Three hours after i.pl. injection of carrageenan, the number of c-Fos-like immunoreactive (c-Fos-LI) neurons was significantly increased in laminae I-II, III-IV and V-VI of the ipsilateral spinal cord at L4-5 with the higher density in laminae I-II and V-VI. I.t. pre-injected IL-1beta significantly decreased the number of carrageenan-induced c-Fos-LI neurons in laminae I-II in the ipsilateral spinal cord and also inhibited the hyperalgesia induced by i.pl. carrageenan. These results suggested that i.t. injection of IL-1beta suppressed the central nociceptive input into laminae I-II and produced an antinociceptive effect.

Sevoflurane suppresses noxious stimulus-evoked expression of Fos-like immunoreactivity in the rat spinal cord via activation of endogenous opioid systems

We investigated the antagonism of sevoflurane antinociception by opioid antagonists in the rat formalin test. Formalin injection into the hindpaw of the rat induces the nocifensive flinching behavior and the expression of Fos-like immunoreactivity (Fos-LI) in the spinal cord. Sevoflurane significantly suppressed the flinching behavior and decreased the number of Fos-LI neurons in the dorsal horn of spinal cord compared with the control group. Moreover, pretreatment with intraperitoneal naloxone plus naltrexone antagonized the suppression of flinching behavior and the decrease of the number of Fos-LI neurons produced by 3% sevoflurane. Intraperitoneal opioid antagonists themselves had no effects on both the behavior response and the expression of Fos-LI induced by formalin injection. This study supports the hypothesis that sevoflurane suppresses the nociceptive response, at least in part, by activating endogenous opioid systems.

The role of nitric oxide in the phosphorylation of cyclic adenosine monophosphate—responsive element-binding protein in the spinal cord after intradermal injection of capsaicin

We investigated the involvement of nitric oxide (NO) in the phosphorylation of cyclic adenosine monophosphate-responsive element-binding protein (CREB) in the spinal cord of rats during central sensitization after intradermal capsaicin injection. CREB and phosphorylated CREB (p-CREB) were measured by immunoblotting. The level of p-CREB increased by 20 minutes, peaked between 20 and 60 minutes after capsaicin injection, and started to decrease after 150 minutes. CREB itself did not show an obvious change after capsaicin injection. The p-CREB expression on the ipsilateral side of the spinal dorsal horn, but not on the contralateral side, increased significantly after capsaicin injection. The increase in p-CREB induced by capsaicin injection was partially blocked by pretreatment with N(G)-nitro-L-arginine methyl ester (L-NAME), an NO synthase inhibitor, administered through a microdialysis fiber placed across the spinal cord. D-NAME, an inactive form of L-NAME, had no effect. CREB phosphorylation, not the level of CREB, was induced within 20 minutes by microdialysis administration of SIN-1, an NO donor. These results indicate that CREB phosphorylation in the spinal cord results from both endogenous and exogenous NO release and that p-CREB may play a role in central sensitization or in longer-term changes in gene expression induced by strong peripheral noxious stimulation.

Exposure of the dorsal root ganglion in rats to pulsed radiofrequency currents activates dorsal horn lamina I and II neurons

OBJECTIVE

Application of pulsed radiofrequency (RF) currents to the dorsal ganglion has been reported to produce long-term relief of spinal pain without causing thermal ablation. The present study was undertaken to identify spinal cord neurons activated by exposure of the dorsal ganglion to pulsed RF currents in rats.

METHODS

Left-sided hemilaminectomy was performed in adult Sprague-Dawley rats to expose the C6 dorsal root ganglion. An RF electrode (0.5 mm diameter) with a thermocouple for temperature monitoring was positioned on the exposed ganglion, and rats were assigned to one of three treatment groups: pulsed RF treatment (20 ms of 500-kHz RF pulses delivered at a rate of 2 Hz for 120 s to produce tissue heated to 38 degrees C), continuous RF (continuous RF currents for 120 s to produce tissue heated to 38 degrees C), or sham treatment (no RF current; electrode maintained in contact with ganglion for 120 s).

RESULTS

Treatment with pulsed RF but not continuous RF was associated with a significant increase in the number of cFOS-immunoreactive neurons in the superficial laminae of the dorsal horn as observed 3 hours after treatment.

CONCLUSION

Exposure of the dorsal ganglion to pulsed RF currents activates pain-processing neurons in the dorsal horn. This effect is not mediated by tissue heating.

Heme oxygenase inhibitors reduce formalin-induced Fos expression in mouse spinal cord tissue

Recent work from our laboratory and others supports a role for heme oxygenase in nociception and pain of several etiologies including inflammatory, incisional and neuropathic. Since it has been observed that heme oxygenase inhibitors reduce formalin-induced pain behaviors in mice and rats, we attempted to determine if this analgesic effect was reflected in a reduction in formalin-induced spinal cord Fos expression, an index of neuronal activation. To perform these studies, it was necessary to first examine the cytoarchitecture of the mouse lumbar spinal cord so that histological sections from known segmental levels could be chosen, and Fos-positive nuclei could be assigned to established dorsal horn laminae. After documenting the segmental and laminar distribution of Fos-positive nuclei following a 5% formalin injection, we went on to determine that the heme oxygenase inhibitor tin-protoporphyrin or morphine reduced this Fos expression as analyzed using confocal fluorescence microscopy. It was also observed that mice lacking expression of heme oxygenase type 2, an isozyme of heme oxygenase found in high abundance in the spinal cord, had lowered Fos expression after the formalin injection. Additional confocal microscopy studies demonstrated widespread expression of heme oxygenase type 2 in spinal cord neuron cell bodies. Double-labeling experiments showed that a high percentage of Fos-positive nuclei identified after administration of formalin were located within heme oxygenase type 2-positive cell profiles. Our studies support the hypothesis that heme oxygenase type 2 plays a role in formalin-induced nociception. Furthermore, from these results we suggest that the heme oxygenase type 2 located in spinal cord dorsal horn neurons participates in this nociceptive pathway.

The effect of paracetamol on nociception and dynorphin A levels in the rat brain

Male Wistar rats were administered with naloxone (1 mg/kg i.p.) or MR 2266 (5 mg/kg i.p) 15 min before paracetamol (400 mg/kg i.p.) treatment and the pain threshold was evaluated. Rats were subjected to the hot-plate and formalin tests and immunoreactive dynorphin A (ir-dynorphin A) levels were measured in the hypothalamus, hippocampus, striatum, brainstem, frontal and parietal-temporal cortex by radioimmunoassay. Pretreatment with naloxone abolished paracetamol antinociceptive activity both in hot-plate and in the first phase, but not in the second phase of the formalin test, while MR 2266 pretreatment was able to antagonise paracetamol effect either in the hot-plate test or in both phases of the formalin test. Among different brain areas investigated paracetamol significantly decreased ir-dynorphin A levels only in the frontal cortex. MR 2266 but not naloxone reversed the decrease in ir-dynorphin A levels elicited by paracetamol. Paracetamol seems to exert its antinociceptive effect also through the opioidergic system modulating dynorphin release in the central nervous system (CNS) of the rat, as suggested by the decrease in the peptide levels.

Identifying pain genes: Bottom-up and top-down approaches

A major goal of pain research at the present time is the identification of pain genes. Such genes have been informally defined in a number of ways, including the deletion or transcriptional inhibition of which produces alterations in behavioral responses on nociceptive assays; those the transcription of which is selective to pain-relevant anatomic loci (eg, small-diameter cells of the dorsal root ganglion); those the transcription of which is enhanced in animals experiencing tonic nociception or hypersensitivity states; and, finally, those existing in polymorphic forms relevant to interindividual variability. The purpose of this review is to compare the utility of various bottom-up and top-down approaches in defining, identifying, and studying pain genes. We will focus on 4 major techniques: transgenic knockouts, antisense knockdowns, gene expression assays (including DNA microarray-based expression profiling), and linkage mapping.

Fos expression is induced by increased nitric oxide release in rat spinal cord dorsal horn

The relationship between exogenous or endogenous nitric oxide and c-fos, an immediate-early gene which can further activate the production of other substances in the central nervous system, was investigated in this study. We found that Fos expression is increased after intradermal capsaicin injection, which also leads to endogenous nitric oxide release in the spinal cord. The increased Fos expression is distributed in neurons of the superficial layers and lamina V of the dorsal horn on the side ipsilateral to the injection. The increased Fos expression is blocked by N(G)-nitro-L-arginine methyl ester, a nitric oxide synthase inhibitor, but not by its inactive isomer N(G)-nitro-D-arginine methyl ester. Fos expression was also increased following the perfusion of 3-morpholino-sydnonimine, a nitric oxide donor, into the dorsal horn through a microdialysis fiber. The increased Fos was distributed within 400 microm from the edge of the microdialysis fiber. Although Fos expression was increased with 3-morpholino-sydnonimine perfusion compared to that seen with artificial cerebrospinal fluid perfusion, there was still some Fos immunostaining in the control sections. Following perfusion of artificial cerebrospinal fluid in the spinal cord of rats pretreated with N(G)-nitro-L-arginine methyl ester, it was found that Fos staining was reduced significantly compared to the control sections from animals without N(G)-nitro-L-arginine methyl ester pretreatment. These results suggest that nitric oxide helps mediate Fos expression induced by an intradermal capsaicin injection. We conclude that both endogenous and exogenous nitric oxide induce Fos expression. Involvement of nitric oxide in the development of central sensitization may affect nociceptive processing by increasing Fos expression. Since many other substances which are related to pain mechanisms can be induced by Fos, it is suggested that nitric oxide may regulate production of these substances through activation of Fos. Nitric oxide is not only involved in the development of central sensitization, but is also involved in the activation of control mechanisms affecting nociception.

Alterations in the distribution of stimulus-evoked c-fos in the spinal cord after neonatal peripheral nerve injury in the rat

Neonatal peripheral nerve injury results in a significant rearrangement of the central terminals of surviving axotomized and adjacent intact primary afferents in the dorsal horn of the spinal cord. This study investigates the ability of these afferents to make functional contacts with dorsal horn cells, using c-fos expression as a marker of synaptic activation. Graded electrical stimulation at A- or C-fiber strength of either the neonatally axotomized sciatic nerve or the adjacent uninjured saphenous nerve was performed in adult rats. Stimulation of the contralateral uninjured nerve served as a control. Quantitative examination of the number and distribution of c-fos-labeled cells in the spinal cord laminae was performed. Electrical stimulation of the previously axotomized sciatic nerve at A-fiber intensity resulted in many labeled profiles in laminae I-V of the lumbar spinal cord on the experimental as compared to the contralateral side. Electrical stimulation of uninjured saphenous nerve or saphenous-nerve-innervated skin (using pin electrodes) at A-fiber intensity did not evoke c-fos. Stimulation of the saphenous nerve at C-fiber intensity, however, resulted in a significant increase in the number and distribution of c-fos-labeled profiles in laminae I-V on the experimental side as compared to the contralateral control side. The results show that the distribution of c-fos-expressing cells after neonatal nerve injury is compatible with the previously demonstrated distribution of sprouting of primary afferents belonging to an uninjured nerve adjacent to an injured nerve, and that the surviving axotomized afferents are capable of transmitting signals to postsynaptic cells. These findings indicate that Abeta afferent stimulation of injured but not uninjured afferents elicits c-fos expression in postsynaptic cells. This may reflect an injury-induced maintenance of a normal developmental process whereby Abeta stimulation elicits c-fos in dorsal horn neurons.

C-fos expression in the spinal cord of rats exhibiting allodynia following contusive spinal cord injury

Contusive spinal cord injury (SCI) may result in central neuropathic pain marked by allodynia-like features in the dermatomes close to the level of injury. The aim of this study was to compare the laminar distribution of activated neurons (as determined by c-fos immediate early gene expression) in the spinal cord immediately above the level of a SCI in rats with or without allodynia-like features. Non-noxious mechanical stimulation was applied to half the animals in the dermatomes corresponding to the level of injury prior to perfusion. Stimulation resulted in a significant increase in c-fos labelling in all laminae of the spinal dorsal horn in the segment immediately above the level of injury only in allodynia animals. Animals that had allodynia also demonstrated a significant increase in the level of c-fos labelling in lamina III, IV and V of the dorsal horn without stimulation. Thus, allodynia following SCI is associated with significant increases in basal and evoked c-fos expression ("neuronal activity") in response to non-noxious mechanical stimulation. The data also suggest that allodynia-like behaviour following SCI cannot be accounted for solely by changes occurring at a spinal level.

Lesions of the dorsolateral funiculus block supraspinal opioid delta receptor mediated antinociception in the rat

Previous experiments have demonstrated that [D-Ala(2), Glu(4)]deltorphin (DELT) produces delta-receptor mediated antinociceptive effects when microinjected into the rat lateral ventricle and ventral medial medullary reticular formation (MRF), but not in the periaqueductal grey region (PAG). The present experiments were undertaken to further characterize the role of delta opioid agonists microinjected into the MRF and to explore the possibility of a descending pain modulatory system which might be linked to supraspinal delta opioid receptors. Rats received formalin into the dorsum of the right hindpaw and flinching responses were recorded. DELT given intracerebroventricularly (i.c.v.), intrathecally (i.th.) or into the MRF before formalin produced a dose-dependent and delta opioid receptor-mediated attenuation of both the first and second phases of the formalin-induced foot flinch response. DELT given i.c.v., i.th., or into the MRF also blocked formalin-induced increase in Fos-like immunoreactivity (FLI) in the dorsal horn of lumbar spinal cord ipsilateral to the formalin injection. Unilateral lesioning of the ipsilateral dorsolateral funiculus (DLF) did not alter nociceptive responses to formalin alone, but blocked the antinociceptive effect of DELT administered into the MRF; DELT was fully active in sham-DLF lesioned rats. Additionally, rats with DLF lesions did not show decreases in formalin-induced FLI in the ipsilateral lumbar spinal cord after injection of DELT into the MRF. These data suggest that delta opioid receptors in the MRF may be involved in activation of a descending inhibitory pain pathway projecting through the DLF to modulate tonic nociceptive input at the spinal level.

Opioidergic and adrenergic modulation of formalin-evoked spinal c-fos mRNA expression and nocifensive behavior in the rat

Fos protein expression has been used to reflect neuronal activation in pain processing pathways although analgesics may uncouple behavioral and Fos responses. We determine whether formalin-induced spinal c-fos mRNA expression (Northern blotting) correlates with nocifensive behavior following pretreatment with morphine, the alpha2-adrenoceptor agonist dexmedetomidine, or their respective antagonists naloxone and atipamezole. Both opiate and alpha2-adrenoceptor agonists reduced formalin-induced c-fos gene transcription and nocifensive behavior via their cognate receptors. Unexpectedly, blockade of either the opiate or alpha2-adrenergic receptors, alone, caused an increase in formalin-evoked c-fos mRNA; while blocking the opiate receptor had no effect on formalin-induced behavior, alpha2-adrenoceptor block had an analgesic effect, indicating discordance between c-fos message transcription and nocifensive behavior. We concluded that the formalin-induced spinal c-fos signal was a poor predictor of the behavioral response to pharmacological manipulation of pain processing pathways.