C-fos expression in rat lumbar spinal cord following peripheral stimulation in adjuvant-induced arthritic and normal rats

Cytokine receptor clustering in sensory neurons with an engineered cytokine fusion protein triggers unique pain resolution pathways

New therapeutic approaches to resolve persistent pain are highly needed. We tested the hypothesis that manipulation of cytokine receptors on sensory neurons by clustering regulatory cytokine receptor pairs with a fusion protein of interleukin (IL)-4 and IL-10 (IL4-10 FP) would redirect signaling pathways to optimally boost pain-resolution pathways. We demonstrate that a population of mouse sensory neurons express both receptors for the regulatory cytokines IL-4 and IL-10. This population increases during persistent inflammatory pain. Triggering these receptors with IL4-10 FP has unheralded biological effects, because it resolves inflammatory pain in both male and female mice. Knockdown of both IL4 and IL10 receptors in sensory neurons in vivo ablated the IL4-10 FP-mediated inhibition of inflammatory pain. Knockdown of either one of the receptors prevented the analgesic gain-of-function of IL4-10 FP. In vitro, IL4-10 FP inhibited inflammatory mediator-induced neuronal sensitization more effectively than the combination of cytokines, confirming its superior activity. The IL4-10 FP, contrary to the combination of IL-4 and IL-10, promoted clustering of IL-4 and IL-10 receptors in sensory neurons, leading to unique signaling, that is exemplified by activation of shifts in the cellular kinome and transcriptome. Interrogation of the potentially involved signal pathways led us to identify JAK1 as a key downstream signaling element that mediates the superior analgesic effects of IL4-10 FP. Thus, IL4-10 FP constitutes an immune-biologic that clusters regulatory cytokine receptors in sensory neurons to transduce unique signaling pathways required for full resolution of persistent inflammatory pain.

The Physiology of Bone Pain. How Much Do We Really Know?

Pain is associated with most bony pathologies. Clinical and experimental observations suggest that bone pain can be derived from noxious stimulation of the periosteum or bone marrow. Sensory neurons are known to innervate the periosteum and marrow cavity, and most of these have a morphology and molecular phenotype consistent with a role in nociception. However, little is known about the physiology of these neurons, and therefore information about mechanisms that generate and maintain bone pain is lacking. The periosteum has received greater attention relative to the bone marrow, reflecting the easier access of the periosteum for experimental assessment. With the electrophysiological preparations used, investigators have been able to record from single periosteal units in isolation, and there is a lot of information available about how they respond to different stimuli, including those that are noxious. In contrast, preparations used to study sensory neurons that innervate the bone marrow have been limited to recording multi-unit activity in whole nerves, and whilst they clearly report responses to noxious stimulation, it is not possible to define responses for single sensory neurons that innervate the bone marrow. There is only limited evidence that peripheral sensory neurons that innervate bone can be sensitized or that they can be activated by multiple stimulus types, and at present this only exists in part for periosteal units. In the central nervous system, it is clear that spinal dorsal horn neurons can be activated by noxious stimuli applied to bone. Some can be sensitized under pathological conditions and may contribute in part to secondary or referred pain associated with bony pathology. Activity related to stimulation of sensory nerves that innervate bone has also been reported in neurons of the spinoparabrachial pathway and the somatosensory cortices, both known for roles in coding information about pain. Whilst these provide some clues as to the way information about bone pain is centrally coded, they need to be expanded to further our understanding of other central territories involved. There is a lot more to learn about the physiology of peripheral sensory neurons that innervate bone and their central projections.

Differential contributions of A- and C-nociceptors to primary and secondary inflammatory hypersensitivity in the rat

There is sensitization to thermal A-nociceptor activation in arthritic secondary hyperalgesia, with enhanced activation of spinal lamina I neurons.

Primary hyperalgesia is characterized by increased responsiveness to both heat and mechanical stimulation in the area of injury. By contrast, secondary hyperalgesia is generally associated with increased responses to mechanical but not heat stimuli. We tested the hypothesis that sensitization in secondary hyperalgesia is dependent on the class of peripheral nociceptor (C- or A-nociceptor) rather than the modality of stimulation (mechanical vs heat). A- and C-nociceptors were selectively activated using contact heat ramps applied to the hind paw dorsum in animals with hind paw inflammation (primary hyperalgesia) and knee inflammatory arthritis (secondary hyperalgesia). Sensitization to A- and C-nociceptor activation in primary and secondary hyperalgesia was assessed by reflex withdrawal thresholds and by Fos immunocytochemistry in the dorsal horn of the spinal cord, as an index of neuronal activation. In primary hyperalgesia, only C-nociceptor-evoked withdrawal reflexes were sensitized. This was associated with increased spinal lamina I neuronal activation to both A- and C-nociceptor activation. Fos-like immunoreactivity (FLI) was unchanged in other dorsal horn laminae. In secondary hyperalgesia, only A-nociceptor-evoked withdrawal reflexes were sensitized, and FLI was increased in both superficial and deep dorsal laminae. Neurons in the superficial dorsal horn receive and process nociceptor inputs from the area of primary hyperalgesia, resulting in functional sensitization to C-nociceptive inputs. In inflammatory arthritis, secondary hyperalgesia is evoked by A-nociceptor thermal stimulation, suggesting that secondary hyperalgesia is A-nociceptor, rather than stimulus modality (mechanical vs thermal), dependent. Fos-like immunoreactivity evoked by A-nociceptor stimulation in secondary hyperalgesia suggests that the sensitization is underpinned by spinal neuronal sensitization in laminae I and IV/V.

Contrasting alterations to synaptic and intrinsic properties in upper-cervical superficial dorsal horn neurons following acute neck muscle inflammation

Background

Acute and chronic pain in axial structures, like the back and neck, are difficult to treat, and have incidence as high as 15%. Surprisingly, most preclinical work on pain mechanisms focuses on cutaneous structures in the limbs and animal models of axial pain are not widely available. Accordingly, we developed a mouse model of acute cervical muscle inflammation and assessed the functional properties of superficial dorsal horn (SDH) neurons.

Results

Male C57/Bl6 mice (P24-P40) were deeply anaesthetised (urethane 2.2 g/kg i.p) and the rectus capitis major muscle (RCM) injected with 40 μl of 2% carrageenan. Sham animals received vehicle injection and controls remained anaesthetised for 2 hrs. Mice in each group were sacrificed at 2 hrs for analysis. c-Fos staining was used to determine the location of activated neurons. c-Fos labelling in carrageenan-injected mice was concentrated within ipsilateral (87% and 63% of labelled neurons in C1 and C2 segments, respectively) and contralateral laminae I - II with some expression in lateral lamina V. c-Fos expression remained below detectable levels in control and sham animals. In additional experiments, whole cell recordings were obtained from visualised SDH neurons in transverse slices in the ipsilateral C1 and C2 spinal segments. Resting membrane potential and input resistance were not altered. Mean spontaneous EPSC amplitude was reduced by ~20% in neurons from carrageenan-injected mice versus control and sham animals (20.63 ± 1.05 vs. 24.64 ± 0.91 and 25.87 ± 1.32 pA, respectively). The amplitude (238 ± 33 vs. 494 ± 96 and 593 ± 167 pA) and inactivation time constant (12.9 ± 1.5 vs. 22.1 ± 3.6 and 15.3 ± 1.4 ms) of the rapid A type potassium current (I_Ar), the dominant subthreshold current in SDH neurons, were reduced in carrageenan-injected mice.

Conclusions

Excitatory synaptic drive onto, and important intrinsic properties (i.e., I_Ar) within SDH neurons are reduced two hours after acute muscle inflammation. We propose this time point represents an important transition period between peripheral and central sensitisation with reduced excitatory drive providing an initial neuroprotective mechanism during the early stages of the progression towards central sensitisation.

Pathophysiological mechanisms of neuropathic pain

Neuropathic pain is a common problem in clinical practice and one that adversely affects patients’ quality of life. Converging evidence from animal and human studies demonstrates that neuropathic pain arises from a lesion in the somatosensory system. Injured peripheral nerve fibers give rise to an intense and prolonged ectopic input to the CNS and, in some cases, also to secondary changes in dorsal horn neuronal excitability. Convincing evidence now suggests that classifying neuropathic pain according to a mechanism-based rather than an etiology-based approach might help in targeting therapy to the individual patient and would be useful in testing new drugs. This article summarizes our current understanding of the peripheral and central pathophysiological mechanisms underlying neuropathic pain and focuses on how symptoms translate into mechanisms.

Patterns of FOS expression in the spinal cord and periaqueductal grey matter of 6OHDA-lesioned rats

A less well-known feature of Parkinson disease is that up to 40% of patients experience distinct sensory disturbances, including hyperalgesia and chronic pain. There is a limited understanding of the neural mechanisms that generate these symptoms, however. This study explores the patterns of Fos expression (a well-known marker for changes in cell activity) in the spinal cord and periaqueductal grey matter (PaG), two major sensory (nociceptive) centers, of hemiParkinsonian rats. The medial forebrain bundle (mfb; major tract carrying dopaminergic nigrostriatal axons) was injected with either 6OHDA or saline (controls). A week later, some rats were subjected to mechanical stimulation (pinching) of the hindpaw for 2 h, whereas others received no stimulation. Thereafter, brains were processed using routine tyrosine hydroxylase (marker for dopaminergic cells) or Fos immunocytochemistry. In the PaG, there were many more Fos(+) cells in the 6OHDA-lesioned than in the Control group, in both the stimulation and, in particular, the non-stimulation cases. In the spinal cord, there were also more Fos(+) cells in the 6OHDA-lesioned than in the Control group, but in the stimulation cases only. Overall, the results show distinct changes in Fos expression in the spinal cord and PaG of 6OHDA-lesioned rats, suggesting a substrate for some of the abnormal sensory (nociceptive) circuits that may be evident in parkinsonian cases.

Peripheral therapeutic ultrasound stimulation alters the distribution of spinal C-fos immunoreactivity induced by early or late phase of inflammation

The purpose of this investigation was to examine the central modulated effects of therapeutic ultrasound (US) on neuronal activity in the spinal cord on early and late phases of inflammation. In this study, induction of c-Fos protein, which reflects neuronal activation (particularly inflammatory nociception), was investigated in the lumbar spinal cord with immunohistochemistry. Inflammatory monoarthritis was induced in 20 male Wistar rats (weighing 250-300 g) via intra-articular injection of complete Freund's adjuvant (CFA) into the tibiotarsal joint. Two phases of arthritis, early phase (18 h after adjuvant injection) and late phase (7 d after adjuvant injection), were studied in the rats. Pulsed-mode US (1 MHz, the spatial average temporal average intensity [I(SATA)] = 0.5 W/cm(2), 50% duty cycle) was applied for 5 min. The effects of US and sham treatments against these phases of arthritis were demonstrated by spinal c-Fos-like immunoreactivity (c-Fos-LI). All data were evaluated statistically with the paired t-test or analysis of variance with Bonferroni corrections. c-Fos-LI neurons were abundant (average 264.2 +/- 11.9) in the L3 and L4 neurons of the spinal cord in areas ipsilateral to the CFA-induced arthritic leg in the early phase, but few were present (average 40.4 +/- 4.5) in the late phase in sham-treated animals. Bonferroni corrections to the alpha level were used to check the group differences in spinal c-Fos expression, and significance was reached when p < 0.025. In the early inflammatory phase, US treatment significantly suppressed the increased number of c-Fos-LI neurons associated with CFA-induced arthritis in superficial laminae, nucleus proprius, deep laminae and ventral horn of the spinal cord. However, during the late inflammatory phase, US significantly triggered c-Fos expression in most laminae, particularly in the nucleus proprius, deep laminae and ventral horn of the spinal cord. The results of our study suggest that administration of US causes a reduction of early nociceptive inflammatory processing, as shown by a decrease in CFA-induced c-Fos-LI neurons at the level of the spinal cord. In contrast, the US did not suppress, but rather enhanced, the number of c-Fos-LI neurons during the late inflammatory phase. The peripheral influences of US on the central modulation of the spinal nociceptive processing system is suggested and may reflect the work being done through the neuroplasticity of spinal cord in response to peripheral stimulation of US. Therefore, we propose a difference in spinal expression of c-Fos-LI neurons between effects of peripheral US stimulation in arthritic models that underlie early and late inflammatory pain. (E-mail: sherrie@sunrise.hk.edu.tw).

Enkephalin-encoding herpes simplex virus-1 decreases inflammation and hotplate sensitivity in a chronic pancreatitis model

Background

A chronic pancreatitis model was developed in young male Lewis rats fed a high-fat and alcohol liquid diet beginning at three weeks. The model was used to assess time course and efficacy of a replication defective herpes simplex virus type 1 vector construct delivering human cDNA encoding preproenkephalin (HSV-ENK).

Results

Most surprising was the relative lack of inflammation and tissue disruption after HSV-ENK treatment compared to the histopathology consistent with pancreatitis (inflammatory cell infiltration, edema, acinar cell hypertrophy, fibrosis) present as a result of the high-fat and alcohol diet in controls. The HSV-ENK vector delivered to the pancreatic surface at week 3 reversed pancreatitis-associated hotplate hypersensitive responses for 4–6 weeks, while control virus encoding β-galactosidase cDNA (HSV-β-gal) had no effect. Increased Fos expression seen bilaterally in pain processing regions in control animals with pancreatitis was absent in HSV-ENK-treated animals. Increased met-enkephalin staining was evident in pancreas and lower thoracic spinal cord laminae I–II in the HSV-ENK-treated rats.

Conclusion

Thus, clear evidence is provided that site specific HSV-mediated transgene delivery of human cDNA encoding preproenkephalin ameliorates pancreatic inflammation and significantly reduces hypersensitive hotplate responses for an extended time consistent with HSV mediated overexpression, without tolerance or evidence of other opiate related side effects.

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.

Treatment of inflamed pancreas with enkephalin encoding HSV-1 recombinant vector reduces inflammatory damage and behavioral sequelae

This study assessed the efficacy of pancreatic surface delivered enkephalin (ENK)-encoding herpes simplex virus type 1 (HSV-1) on spontaneous behaviors and spinal cord and pancreatic enkephalin expression in an experimental pancreatitis model. Replication-defective HSV-1 with proenkephalin complementary DNA (cDNA) (HSV-ENK) or control beta-galactosidase cDNA (HSV-beta-gal), or media vehicle (Veh) was applied to the pancreatic surface of rats with dibutyltin dichloride (DBTC)-induced pancreatitis. Spontaneous exploratory behavioral activity was monitored on days 0 and 6 post DBTC and vector treatments. The pancreas, thoracic dorsal root ganglia (DRG, T9-10), and spinal cord (T9-10) were immunostained for met-enkephalin (met-ENK), beta-gal, and HSV-1 proteins. Spinal cord was also immunostained for c-Fos, and pancreas was stained for the inflammatory marker regulated on activation, normal T-cells expressed and secreted (RANTES), mu-opioid receptor, and hemotoxylin/eosin. On day 6, compared to pancreatitis and vector controls, the DBTC/HSV-ENK treated rats had significantly improved spontaneous exploratory activities, increased met-ENK staining in the pancreas and spinal cord, and normalized c-Fos staining in the dorsal horn. Histopathology of pancreas in DBTC/HSV-ENK treated rats showed preservation of acinar cells and cytoarchitecture with minimal inflammatory cell infiltrates, compared to severe inflammation and acinar cell loss seen in DBTC/HSV-beta-gal and DBTC/Veh treated rats. Targeted transgene delivery and met-ENK expression successfully produced decreased inflammation in experimental pancreatitis.

Spinal NK-1 receptor expressing neurons mediate opioid-induced hyperalgesia and antinociceptive tolerance via activation of descending pathways

Opioids can induce hyperalgesia in humans and in animals. Mechanisms of opiate-induced hyperalgesia and possibly of spinal antinociceptive tolerance may be linked to pronociceptive adaptations occurring at multiple levels of the nervous system including activation of descending facilitatory influences from the brainstem, spinal neuroplasticity, and changes in primary afferent fibers. Here, the role of NK-1 receptor expressing cells in the spinal dorsal horn in morphine-induced hyperalgesia and spinal antinociceptive tolerance was assessed by ablating these cells with intrathecal injection of SP-saporin (SP-SAP). Ablation of NK-1 receptor expressing cells prevented (a) morphine-induced thermal and mechanical hypersensitivity, (b) increased touch-evoked spinal FOS expression, (c) upregulation of spinal dynorphin content and (d) the rightward displacement of the spinal morphine antinociceptive dose-response curve (i.e., tolerance). Morphine-induced hyperalgesia and antinociceptive tolerance were also blocked by spinal administration of ondansetron, a serotonergic receptor antagonist. Thus, NK-1 receptor expressing neurons play a critical role in sustained morphine-induced neuroplastic changes which underlie spinal excitability reflected as thermal and tactile hypersensitivity to peripheral stimuli, and to reduced antinociceptive actions of spinal morphine (i.e., antinociceptive tolerance). Ablation of these cells likely eliminates the ascending limb of a spinal-bulbospinal loop that engages descending facilitation and elicits subsequent spinal neuroplasticity. The data may provide a basis for understanding mechanisms of prolonged pain which can occur in the absence of tissue injury.

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.

GFAP and Fos immunoreactivity in lumbo-sacral spinal cord and medulla oblongata after chronic colonic inflammation in rats

AIM: To investigate the response of astrocytes and neurons in rat lumbo-sacral spinal cord and medulla oblongata induced by chronic colonic inflammation, and the relationship between them.

METHODS: Thirty-three male Sprague-Dawley rats were randomly divided into two groups: experimental group (n = 17), colonic inflammation was induced by intra-luminal administration of trinitrobenzenesulfonic acid (TNBS); control group (n = 16), saline was administered intra-luminally. After 3, 7, 14, and 28 d of administration, the lumbo-sacral spinal cord and medulla oblongata were removed and processed for anti-glial fibrillary acidic protein (GFAP), Fos and GFAP/Fos immunohistochemistry.

RESULTS: Activated astrocytes positive for GFAP were mainly distributed in the superficial laminae (laminae I-II) of dorsal horn, intermediolateral nucleus (laminae V), posterior commissural nucleus (laminae X) and anterolateral nucleus (laminae IX). Fos-IR (Fos-immunoreactive) neurons were mainly distributed in the deeper laminae of the spinal cord (laminae III-IV, V-VI). In the medulla oblongata, both GFAP-IR astrocytes and Fos-IR neurons were mainly distributed in the medullary visceral zone (MVZ). The density of GFAP in the spinal cord of experimental rats was significantly higher after 3, 7, and 14 d of TNBS administration compared with the controls (50.4±16.8, 29.2±6.5, 24.1±5.6, P<0.05). The density of GFAP in MVZ was significantly higher after 3 d of TNBS administration (34.3±2.5, P<0.05). After 28 d of TNBS administration, the density of GFAP in the spinal cord and MVZ decreased and became comparable to that of the controls (18.0±4.9, 14.6±6.4, P>0.05).

CONCLUSION: Astrocytes in spinal cord and medulla oblongata can be activated by colonic inflammation. The activated astrocytes are closely related to Fos-IR neurons. With the recovery of colonic inflammation, the activity of astrocytes in the spinal cord and medulla oblongata is reduced.

Bone cancer pain: from mechanism to model to therapy

Although bone cancer pain can be severe and is relatively common, very little is known about the basic mechanisms that generate and maintain this debilitating pain. To begin to define the mechanisms that give rise to bone cancer pain, a mouse model was developed using the intramedullary injection and containment of osteolytic sarcoma cells in the mouse femur. These tumor cells induced bone destruction as well as ongoing and movement-evoked pain behaviors similar to that found in patients with bone cancer pain. In addition, there was a significant reorganization of the spinal cord that received sensory input from the cancerous bone, and this reorganization was significantly different from that observed in mouse models of chronic neuropathic or inflammatory pain. To determine whether this mouse model of bone cancer could be used to define the basic mechanisms giving rise to bone cancer pain, we targeted excessive osteoclast activity using osteoprotegerin, a secreted decoy receptor that inhibits osteoclast activity. Osteoprotegerin blocked excessive tumor-induced, osteoclast-mediated bone destruction, and significantly reduced ongoing and movement-evoked pain, and the neurochemical reorganization of the spinal cord. These data suggest that this model can provide insight into the mechanisms that generate bone cancer pain and provide a platform for developing and testing novel analgesics to block bone cancer pain.

Suckling-induced activation of neural c-fos expression at lower thoracic rat spinal cord segments

Suckling stimulation is essential for neuroendocrine and sympathetic reflex activation during lactation. In the present study, the induction of c-fos gene expression was used to identify neuronal populations in the spinal cord activated by acute 5 min suckling or by electrical stimulation of the central stump of the first abdominal mammary nerve in lactating rats previously separated from their litters for 6 or 18 h. In addition, to investigate whether spinal sympathetic preganglionic neurons are activated by suckling, dual immunostaining (Fos and choline acetyltransferase) was performed. Fos was expressed at low levels in continuously suckled and 6 h nonsuckled mothers, but no expression was found after 18 h of nonsuckling. On the other hand, in 6 h nonsuckled rats, significant increments in Fos expression occurred in several regions after acute suckling and after electrical stimulation. Also, the pattern of Fos expression in each spinal laminae was different for the two stimuli, i.e. more intense effects of suckling in deep laminae V-X and more intense effects in laminae I-IV with electrical stimulation. Double-labeling after suckling was found only in sympathetic preganglionic neurons from the intermedio-medial cell column, whereas after electrical stimulation, double label was observed only in neurons from the intermedio-lateral cell column. On the other hand, no effect upon Fos protein expression was observed after suckling and only a minor effect after electrical stimulation of mammary nerve in 18 h nonsuckled rats. These results are consistent with previous findings on the sympathetic reflex regulation of the mammary gland, as well as on the importance of the nonsuckling interval for optimal functioning of lactation.

BDNF: a neuromodulator in nociceptive pathways?

During development, brain-derived neurotrophic factor (BDNF) supports the survival of certain neuronal population in central and peripheral nervous system. In adulthood, BDNF has been suggested to act as an important modulator of synaptic plasticity. This article reviews and discusses its potential role as neuromodulator in the spinal dorsal horn. BDNF is synthesized in the cell body of primary sensory neurons (pre-synaptic neurons) and its expression is regulated in models of inflammatory and neuropathic pain. The high affinity receptor for BDNF, tropomyosine receptor kinase B (TrkB), is expressed by post-synaptic neurons of the dorsal horn. Stimulation of pre-synaptic nociceptive afferent fibres induces BDNF release and consequent activation of TrkB receptors leading to a post-synaptic excitability. Electrophysiological recordings showed that BDNF enhances the ventral root potential induced by C-fibre stimulation in an in vitro preparation. In addition, behavioural data indicate that antagonism of BDNF attenuates the second phase of hyperalgesia induced by formalin (in nerve growth factor-treated animals) and the thermal hyperalgesia induced by carageenan, suggesting that BDNF is involved in some aspects of central sensitisation in conditions of peripheral inflammation. In conclusion, BDNF meets many of the criteria necessary to define it as a neurotransmitter/neuromodulator in small diameter nociceptive neurons.

Trigeminal c-Fos expression and behavioral responses to pulpal inflammation in ferrets

Injury to peripheral dental tissues evokes dynamic alternations in central sensory pathways. We have previously reported that transient stimulation of the dental pulp with noxious heat evokes the induction of the immediate early gene product Fos in the transitional region between subnucleus interpolaris and caudalis (Vi/Vc) and subnucleus caudalis (Vc). A question arises as to whether similar changes occur in response to inflammation to the tooth pulp. In this study, the effects of pulpal inflammation produced by bacterial lipopolysaccharide (LPS) on face-grooming behavior and trigeminal Fos expression were examined. Face-grooming behaviors were recorded daily for 3 days pre- and 24, 48 and 72 h post- LPS or saline application. All animals were perfused 72 h post- LPS or saline application. Brainstems were processed for Fos-like immunoreactivity (Fos-LI). Teeth were processed for H&E staining. Histological examination of LPS-treated teeth revealed features of an acute pulpitis. Moreover, LPS-treated animals showed greater face-grooming activity (i.e. tongue protrusions) directed to the injured tooth than the sham-operated group. The number of Fos-positive neurons was greater in the trigeminal subnucleus caudalis (Vc) and the transitional regions (Vi/Vc) in LPS-treated animals compared with sham-operated animals, and greater in the deeper laminae than the superficial laminae of each trigeminal region. LPS treatment did not evoke Fos expression in the rostral trigeminal regions above Vi/Vc. These results demonstrate that LPS-induced pulpal inflammation results in significant alterations in the Vi/Vc and Vc, and such changes may underlie the observed nociceptive behavioral responses and may play an important role in producing a symptomatic pulpitis in humans.

Transient spinal cord ischemia in rat: the time course of spinal FOS protein expression and the effect of intraischemic hypothermia (27 degrees C)

1. In the present study, we characterize the time course of spinal FOS protein expression after transient noninjurious (6-min) or injurious (12-min) spinal ischemia induced by inflation of a balloon catheter placed into the descending thoracic aorta. In addition, this work examined the effects of spinal hypothermia on FOS expression induced either by ischemia or by potassium-evoked depolarization (intrathecal KCl). 2. Short-lasting (6-min) spinal ischemia evoked a transient FOS protein expression. The peak expression was seen 2 hr after reperfusion in all laminar levels in lumbosacral segments. At 4 hr of reperfusion, more selective FOS expression in spinal interneurons localized in the central part of laminae V-VII was seen. At 24 hr no significant increase in FOS protein was detected. 3. After 12 min of ischemia and 2 hr of reflow, nonspecific FOS expression was seen in both white and gray matter, predominantly in nonneuronal elements. Intrathecal KCl-induced FOS expression in spinal neurons in the dorsal horn and in the intermediate zone. Spinal hypothermia (27 degrees C) significantly suppressed FOS expression after 6 or 12 min of ischemia but not after KCl-evoked depolarization. 4. Data from the present study show that an injurious (but not noninjurious) interval of spinal ischemia evokes spinal FOS protein expression in glial cells 2 hr after reflow. The lack of neuronal FOS expression corresponds with extensive neuronal degeneration seen in this region 24 hr after reflow. Noninjurious (6-min) ischemia induced a transient, but typically neuronal FOS expression. The significant blocking effect of hypothermia (27 degrees C) on the FOS induction after ischemia but not after potassium-evoked depolarization also suggests that simple neuronal depolarization is a key trigger in FOS induction.

Osteoprotegerin blocks bone cancer-induced skeletal destruction, skeletal pain and pain-related neurochemical reorganization of the spinal cord

Bone cancer pain is common among cancer patients and can have a devastating effect on their quality of life. A chief problem in designing new therapies for bone cancer pain is that it is unclear what mechanisms drive this distinct pain condition. Here we show that osteoprotegerin, a secreted 'decoy' receptor that inhibits osteoclast activity, also blocks behaviors indicative of pain in mice with bone cancer. A substantial part of the actions of osteoprotegerin seems to result from inhibition of tumor-induced bone destruction that in turn inhibits the neurochemical changes in the spinal cord that are thought to be involved in the generation and maintenance of cancer pain. These results demonstrate that excessive tumor-induced bone destruction is involved in the generation of bone cancer pain and that osteoprotegerin may provide an effective treatment for this common human condition.

Neurochemical and cellular reorganization of the spinal cord in a murine model of bone cancer pain

The cancer-related event that is most disruptive to the cancer patient's quality of life is pain. To begin to define the mechanisms that give rise to cancer pain, we examined the neurochemical changes that occur in the spinal cord and associated dorsal root ganglia in a murine model of bone cancer. Twenty-one days after intramedullary injection of osteolytic sarcoma cells into the femur, there was extensive bone destruction and invasion of the tumor into the periosteum, similar to that found in patients with osteolytic bone cancer. In the spinal cord, ipsilateral to the cancerous bone, there was a massive astrocyte hypertrophy without neuronal loss, an expression of dynorphin and c-Fos protein in neurons in the deep laminae of the dorsal horn. Additionally, normally non-noxious palpation of the bone with cancer induced behaviors indicative of pain, the internalization of the substance P receptor, and c-Fos expression in lamina I neurons. The alterations in the neurochemistry of the spinal cord and the sensitization of primary afferents were positively correlated with the extent of bone destruction and the growth of the tumor. This "neurochemical signature" of bone cancer pain appears unique when compared to changes that occur in persistent inflammatory or neuropathic pain states. Understanding the mechanisms by which the cancer cells induce this neurochemical reorganization may provide insight into peripheral factors that drive spinal cord plasticity and in the development of more effective treatments for cancer pain.

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.

Emerging concepts in the neurobiology of chronic pain: evidence of abnormal sensory processing in fibromyalgia

Chronic pain often differs from acute pain. The correlation between tissue pathology and the perceived severity of the chronic pain experience is poor or even absent. Furthermore, the sharp spatial localization of acute pain is not a feature of chronic pain; chronic pain is more diffuse and often spreads to areas beyond the original site. Of importance, chronic pain seldom responds to the therapeutic measures that are successful in treating acute pain. Physicians who are unaware of these differences may label the patient with chronic pain as being neurotic or even a malingerer. During the past decade, an exponential growth has occurred in the scientific underpinnings of chronic pain states. In particular, the concept of nonnociceptive pain has been refined at a physiologic, structural, and molecular level. This review focuses on this new body of knowledge, with particular reference to the chronic pain state termed "fibromyalgia."

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.

Inducible and constitutive transcription factors in the mammalian nervous system: control of gene expression by Jun, Fos and Krox, and CREB/ATF proteins

This article reviews findings up to the end of 1997 about the inducible transcription factors (ITFs) c-Jun, JunB, JunD, c-Fos, FosB, Fra-1, Fra-2, Krox-20 (Egr-2) and Krox-24 (NGFI-A, Egr-1, Zif268); and the constitutive transcription factors (CTFs) CREB, CREM, ATF-2 and SRF as they pertain to gene expression in the mammalian nervous system. In the first part we consider basic facts about the expression and activity of these transcription factors: the organization of the encoding genes and their promoters, the second messenger cascades converging on their regulatory promoter sites, the control of their transcription, the binding to dimeric partners and to specific DNA sequences, their trans-activation potential, and their posttranslational modifications. In the second part we describe the expression and possible roles of these transcription factors in neural tissue: in the quiescent brain, during pre- and postnatal development, following sensory stimulation, nerve transection (axotomy), neurodegeneration and apoptosis, hypoxia-ischemia, generalized and limbic seizures, long-term potentiation and learning, drug dependence and withdrawal, and following stimulation by neurotransmitters, hormones and neurotrophins. We also describe their expression and possible roles in glial cells. Finally, we discuss the relevance of their expression for nervous system functioning under normal and patho-physiological conditions.

A capsaicin-receptor antagonist, capsazepine, reduces inflammation-induced hyperalgesic responses in the rat: evidence for an endogenous capsaicin-like substance

In the present study, the presence of an endogenous capsaicin-like substance and the role of capsaicin receptors in nociception during inflammation were assessed using Fos immunohistochemistry and the paw-withdrawal test in rats. Intradermal injection of carrageenan in the hind-paw produced inflammation in the foot pad, increased the number of cells exhibiting Fos-like immunoreactivity in the dorsal horn of the spinal cord, and decreased the paw-withdrawal latency. Intradermal injection of capsazepine, a capsaicin-receptor antagonist, significantly reduced the number of cells exhibiting Fos-like immunoreactivity, significantly increased the paw-withdrawal latency, but did not decrease inflammation induced by carrageenan injection. Intradermal injection of capsaicin or formalin also increased Fos-positive neurons. Capsaicin- or formalin-induced Fos expression was reduced in both cases by pretreatment of capsazepine, but to a much lesser extent for formalin. The capsazepine inhibition of carrageenan inflammation-induced hyperalgesic responses strongly suggests that an endogenous capsaicin-like substance is released in inflamed tissues and produces nociceptive neural impulses by acting on capsaicin receptors present on sensory neurons. Furthermore, our results indicate that capsaicin receptors take part only in generating nociceptive signals in sensory neurons, but not in activating the inflammation-promoting cells.

Suckling and sucrose ingestion suppress persistent hyperalgesia and spinal Fos expression after forepaw inflammation in infant rats

Sweet taste and nonnutritive suckling produce analgesia to transient noxious stimuli in infant rats and humans. The present study evaluated the pain-modulating effects of sucrose and suckling in a rat model of persistent pain and hyperalgesia that mimics the response to tissue injury in humans. Fore- and hindpaw withdrawal latencies from a 30 degrees or 48 degrees C brass stylus were determined in 10-day-old rats following paw inflammation induced by complete Freund's adjuvant (CFA; 1:1 injected s.c. in a 0.01 ml volume). CFA markedly decreased escape latencies to both 48 degrees and 30 degrees C stimulation, thereby demonstrating thermal hyperalgesia and mechanical allodynia. The combination of nonnutritive suckling and sucrose (7.5%, 0.01-0.06 ml/min) infusion markedly increased escape latencies to forepaw stimulation in both CFA-treated and control rats. In contrast, intraoral sucrose and suckling did not increase hindpaw withdrawal latencies in either control or CFA-inflamed rats. The effect was specific to sweet taste because neither water nor isotonic saline infusion affected forepaw escape latencies. Parallel findings were obtained for CFA-induced Fos-like immunoreactivity (Fos-LI), a marker of neuronal activation. Fos-LI was selectively induced in cervical and lumbar regions ipsilateral to forepaw and hindpaw inflammation, respectively. Suckling-sucrose treatment significantly reduced Fos-LI at the cervical but not at the lumbar regions. These findings demonstrate: (i) the development of persistent pain and hyperalgesia in 10-day-old rats that can be attenuated by endogenous pain-modulating systems activated by taste and nonnutritive suckling; (ii) the mediation of the sucrose-suckling analgesia and antihyperalgesia at the spinal level; and (iii) a differential rostrocaudal maturation of descending pain-modulating systems to the spinal cord of 10-day-old rats. These findings may provide new clinical approaches for engaging endogenous analgesic mechanisms in infants following tissue injury and inflammation.

Basal and touch-evoked fos-like immunoreactivity during experimental inflammation in the rat

Fos-immunoreactivity can readily be induced in spinal cord neurones by noxious, but to a much more limited extent, by innocuous peripheral stimuli. The present study has investigated whether low intensity stimuli and electrical stimulation of A beta afferents elicit greater c-fos expression during the behavioural sensory hypersensitivity generated by experimental peripheral inflammation. We have examined the time-course of c-fos expression after inflammation produced by either an intra-plantar injection of the irritant turpentine oil or of complete Freund's adjuvant (CFA). In the former case, a significant initial expression in all dorsal horn laminae was followed by a gradual decrease, whereas after CFA injection, an initial expression limited to the superficial laminae subsequently extended into the deep laminae, with a decrease at 24 h and an increase in labelling at later times. Low intensity touch stimuli repeated for 10 min, when applied at 24 h and 48 h after CFA injection, elicited a significant increase in the number of Fos-immunoreactive neurons in both the superficial and deep laminae of the dorsal horn compared to non-inflamed animals. Electrical stimulation of the sciatic nerve 24 h post-CFA injection, at a strength sufficient only to activate A beta-afferents fibres (100 microA, 50 microseconds, 10 min), also elicited a significant increase in labelling relative to the same stimuli applied in control animals, especially in laminae V-VI. The present results demonstrate that low intensity cutaneous stimuli elicit a significantly greater increase in c-fos expression in dorsal horn neurons during peripheral inflammation and that A beta-afferent input contributes to this, a finding that may relate to the allodynia experienced during inflammation.

Differential c-fos expression in the nucleus of the solitary tract and spinal cord following noxious gastric distention in the rat

c-Fos has been used as a marker for activity in the spinal cord following noxious somatic or visceral stimulation. Although the viscera receive dual afferent innervation, distention of hollow organs (i.e. esophagus, stomach, descending colon and rectum) induces significantly more c-Fos in second order neurons in the nucleus of the solitary tract and lumbosacral spinal cord, which receive parasympathetic afferent input (vagus, pelvic nerves), than the thoracolumbar spinal cord, which receives sympathetic afferent input (splanchnic nerves). The purpose of this study was to determine the contribution of sympathetic and parasympathetic afferent input to c-Fos expression in the nucleus of the solitary tract and spinal cord, and the influence of supraspinal pathways on Fos induction in the thoracolumbar spinal cord. Noxious gastric distention to 80 mmHg (gastric distension/80) was produced by repetitive inflation of a chronically implanted gastric balloon. Gastric distension/80 induced c-Fos throughout the nucleus of the solitary tract, with the densest labeling observed within 300 microns of the rostral pole of the area postrema. This area was analysed quantitatively following several manipulations. Gastric distension/80 induced a mean of 724 c-Fos-immunoreactive nuclei per section. Following subdiaphragmatic vagotomy plus distention (vagotomy/80), the induction of c-Fos-immunoreactive nuclei was reduced to 293 per section, while spinal transection at T2 plus distention (spinal transection/80) induced a mean of 581 nuclei per nucleus of the solitary tract section. Gastric distension/80 and vagotomy/80 induced minimal c-Fos in the T8-T10 spinal cord (50 nuclei/section), but spinal transection/80 induced 200 nuclei per section. Repetitive bolus injections of norepinephrine produced transient pressor responses mimicking the pressor response produced by gastric distension/80. This manipulation induced minimal c-Fos in the nucleus of the solitary tract and none in the spinal cord. It is concluded that noxious visceral input via parasympathetic vagal afferents, and to a lesser extent sympathetic afferents and the spinosolitary tract, contribute to gastric distention-induced c-Fos in the nucleus of the solitary tract. The induction of c-Fos in the nucleus of the solitary tract is significantly greater than in the viscerotopic segments of the spinal cord, which is partially under tonic descending inhibition, but is not subject to modulation by vagal gastric afferents. Distention pressures produced by noxious gastric distention are much greater than those produced during feeding, suggesting that c-Fos induction in the nucleus of the solitary tract to noxious distention is not associated with physiological mechanisms of feeding and satiety. The large vagal nerve-mediated induction of c-Fos in the nucleus of the solitary tract following gastric distension suggests that parasympathetic afferents contribute to the processing of noxious visceral stimuli, perhaps by contributing to the affective-emotional component of visceral pain.

The suppression of formalin-induced fos expression by different anesthetic agents in the infant rat

Despite the importance of pediatric anesthesiology, the sites and mechanisms of anesthetic action in the neonate are not well described in either human or nonhuman species. This experiment investigated suppression produced by different anesthetic agents of neuronal activity in the lumbar spinal cord of the 3-day-old rat. The expression of the c-fos immediate early gene following formalin injection into the hindpaw was used as a marker for neuronal activity. Pups were anesthetized by one of the following often-used agents: methoxyflurane, acepromazine, a mixture of ketamine and xylazine, and hypothermia. All treatments induced behavioral anesthesia. Despite the behavioral anesthesia, the ketamine-xylazine mixture was completely ineffective in suppressing formalin-induced-Fos expression. In contrast, methoxyflurane and hypothermia blocked the appearance of the Fos protein. Similarly, acepromazine was effective in eliminating some of the Fos-labeled nuclei. These data suggest that, in the infant rat, both hypothermia and methoxyflurane act in part at the spinal level by depressing either primary afferents or dorsal horn neuronal activity whereas the site of action for ketamine-xylazine may be located supraspinally.

Expression of mu- and kappa-, but not delta-, opioid receptor mRNAs is enhanced in the spinal dorsal horn of the arthritic rats

The expression of the mRNAs for mu-, delta- and kappa-opioid receptors was studied in the lumbar spinal cord of the rats with the inflammation at their unilateral hindpaw using in situ hybridization technique. On 11 days after the first adjuvant inoculation, mu- and kappa-opioid receptor mRNA levels in laminae I-II of the spinal dorsal horn ipsilateral to the inflamed hindpaw were increased to 135.3 +/- 6.4% and 130.3 +/- 5.7%, respectively, compared with the contralateral side. At this time point, no significant differences in the mu- and kappa-opioid receptor mRNA expression were observed between ipsi- and contralateral sides of other laminae. On the other hand, no significant change was observed in the delta-opioid receptor mRNA expression throughout the laminae I-IX at any time points examined. These findings suggest the increase in the synthesis of mu- and kappa-, but not delta-, opioid receptors in the spinal laminae I-II during sustained inflammatory pain.

Fos expression in the medullary dorsal horn of the rat after chronic constriction injury to the infraorbital nerve

Chronic constriction injury to the rat's infraorbital nerve (IoN-CCI) induces asymmetric face grooming directed to the injured nerve territory and, beginning at 7-12 days postoperative, hyperresponsiveness to mechanical stimulation in this territory (B.P. Vos, A.M. Strassman, and R.J. Maciewicz, 1994, J. Neurosci. 14:2708-2723). To examine central mechanisms involved in these behavioral alterations, changes in nonevoked and mechanical stimulation-evoked fos-like immunoreactivity (fos-LI) following IoN-CCI were quantified in the medullary dorsal horn. Following the appearance of hyperresponsiveness in IoN-CCI rats, experimental and matched sham-operated rats were anesthetized with urethane and received either no stimulation or repeated stimulation with either a 2- or 15-g von Frey hair applied to the hairy skin between vibrissae B3-4/C3-4 on the operated side. Unstimulated IoN-CCI rats had increased fos-LI in laminae I-IV of the ipsilateral medullary dorsal horn. In both groups, mechanical stimulation produced a distinct pattern of fos-LI in the ipsilateral medullary dorsal horn, the quantity of which was related to stimulus intensity. For both stimulus intensities, the total amount and the rostrocaudal spread of evoked fos-LI were significantly larger in IoN-CCI rats. In IoN-CCI rats, stimulation-evoked increases in fos-LI were proportionally larger in laminae I-II than in III-IV. This laminar effect was also present in sham-operated rats but only for 15-g stimulation. Neither condition nor stimulus intensity affected fos-LI in the contralateral medullary dorsal horn. Positive correlations were found between the behavioral parameters of increased trigeminal nociceptive activity and the total amount of fos-LI in the ipsilateral medullary dorsal horn. The results demonstrate that IoN-CCI induces significant alterations in the central processing of afferent signals, which may underlie behavioral manifestations of increased nociceptive activity.

Adrenal medullary grafts suppress c-fos induction in spinal neurons of arthritic rats

Expression of immediate-early genes such as c-fos is thought to reflect patterns of neuronal activity in the central nervous system. Prolonged increases in Fos-protein-like-immunoreactivity (FOS-LI) are seen in the dorsal horn of adjuvant arthritic rats, and parallel increased pain behavior. Grafts of adrenal medullary, but not control tissue, into the spinal subarachnoid space reduce pain behavior and suppress the induction of spinal Fos-LI in arthritic rats. This reduction is particularly marked in superficial laminae (I-II), but is also significant in deeper laminae (III-IV and V-VI). The results of this study suggest that adrenal medullary transplants reduce spinal cord hyperactivation consequent to painful peripheral inflammation.

Distribution of Fos-like immunoreactivity in the caudal brainstem of the rat following noxious chemical stimulation of the temporomandibular joint

Central expression of the protooncogene c-fos was used to examine areas receiving noxious sensory input from the rat temporomandibular joint (TMJ). Fos-like immunoreactivity (Fos-LI) in the caudal brainstem was visualized 2 hours after unilateral injection of the small-fiber-specific excitant/inflammatory irritant mustard oil into the TMJ region. Control animals received injection of either mustard oil into the subcutaneous fascia overlying the masseter muscle or mineral oil vehicle into the TMJ region. In all groups, Fos-LI was consistently observed ipsilaterally in the spinal trigeminal nucleus and cervical dorsal horn and, bilaterally, in the nucleus of the solitary tract and the ventrolateral medulla. The expression of Fos-LI ipsilaterally in the paratrigeminal nucleus was variable. Within the trigeminal sensory complex, Fos-LI was restricted to subnucleus caudalis and the caudal portions of subnucleus interpolaris near the level of the obex. Approximately 12% of Fos-LI cells in subnucleus caudalis and in the cervical dorsal horn were found in laminae III-VI. Compared to TMJ mustard oil injection, mineral oil injection produced less Fos-LI at all rostrocaudal levels, whereas subcutaneous mustard oil injection produced less Fos-LI in caudal subnucleus caudalis but similar amounts in the cervical dorsal horn. Neither of these injections yielded significant ipsilateral responses in subnucleus caudalis, indicating that Fos-LI in this region following TMJ mustard oil injection could be ascribed solely to small-fiber stimulation in the deep TMJ region. The wide rostrocaudal distribution of Fos-LI within the caudal brainstem reflects the distribution of TMJ-responsive nociceptive neurons that may underlie the spread and referral of pain from the TMJ region.

Morphine, 5-HT2 and 5-HT3 receptor antagonists reduce c-fos expression in the trigeminal nuclear complex following noxious chemical stimulation of the rat nasal mucosa

Noxious chemical stimulation of the rat nasal mucosa induces the expression of the immediate early gene c-fos in trigeminal brainstem neurons. In the present study, we applied the irritant mustard oil (1%) into the left nostril of urethane anesthetized rats. Immunohistochemical methods were used to evaluate the expression of Fos protein in the trigeminal subnuclei interpolaris and caudalis and to test the effects of putative analgesics that might depress synaptic transmission in neurons related to nociception. For this purpose, morphine (3 mg/kg and 10 mg/kg), the 5-HT2 antagonist ketanserin (0.5 mg/kg and 5 mg/kg) and the 5-HT3 antagonist ICS 205-930 (0.1 mg/kg and 1 mg/kg) were administered intravenously prior to noxious stimulation. Pretreatment with any of the three compounds reduced Fos-like immunoreactivity. The effect of morphine was reversible with naloxone. The reduction of the expression of Fos-like immunoreactivity by exogenous morphine speaks in favour of an opioidergic link in the modulation of orofacial pain in the trigeminal nuclei. The effects of the 5-HT receptor antagonists are most likely mediated via 5-HT2 and 5-HT3 receptors located on primary afferent fibres.

Carrageenin-evoked c-Fos expression in rat lumbar spinal cord: the effects of indomethacin

This study evaluated the effects of systemic indomethacin on carrageenin evoked c-Fos expression in rat lumbar spinal cord neurons. Fos-like immunoreactivity was not observed after the intraplantar injection of the control vehicle saline. 2 h after administration of carrageenin (6 mg/150 microliters) into the hind limb, Fos-like immunoreactive neurons were observed in the lumbar spinal cord (64 labelled neurons per L4-L5 sections) and were numerous in the superficial laminae (I-II), whereas at 3-4 h both superficial and deeper laminae (V, VI and ventral horn) were labelled. 3 h after carrageenin administration, maximal Fos-like immunoreactivity was observed (104 labelled neurons per L4-L5 sections). At later time points Fos-like immunoreactivity was observed predominantly in the deeper laminae. Fos-like immunoreactivity was rarely observed within laminae III-IV at any of the time points. At 24 h, the number of Fos-like immunoreactive neurons decreased (36 labelled neurons per L4-L5 sections). With increasing doses of carrageenin, an increase in the number of Fos-like immunoreactive neurons was observed. The number of Fos-like immunoreactive neurons induced by the carrageenin stimulation (6 mg, at 3 h) was clearly reduced by oral pretreatment with indomethacin (20 mg/kg). In addition, i.v. indomethacin (1, 2.5 or 5 mg/kg) dose dependently reduced the number of Fos-like immunoreactive neurons and the inflammation of the paw and the ankle of the injected foot. A strong relationship between the effect of indomethacin on c-Fos expression and its effect on inflammatory processes was observed. These results suggest that Fos-like immunoreactivity induced by carrageenin inflammation may be a very useful tool to study the effects of anti-inflammatory drugs, at both peripheral and central levels of inflammation.

Induction of fos-like immunoreactivity by electrocutaneous stimulation of the rat hindpaw

Stimulation of peripheral nerves activates the proto-oncogene c-fos, which in turn generates its gene product, Fos. Fos and Fos-like proteins are produced in the central nervous system in response to chemical, mechanical, thermal, and electrical manipulation. The present study demonstrated a relationship between the number of Fos-like-immunoreactive nuclei in the spinal dorsal horn and graded intensities of electrical stimulation applied to the hindpaws of anesthetized and unanesthetized rats. Stimulation levels within the range of 0.1 to 1.0 mA were chosen on the basis of parmeters previously determined in behavioral investigations of escape reactions. Focal stimulation at these intensities activates peripheral axons directly, but does not injure or traumatize peripheral tissues. There was no evidence of inflammation or edema as a result of the focal electrical stimulation. As the stimulation intensity increased, the number and distribution of Fos-like-labeled nuclei increased with respect to rostral-caudal and laminar orientation. The threshold for expression of Fos-like immunoreactivity was different for anesthetized and unanesthetized animals. For anesthetized animals, the number of labeled nuclei increased significantly from the control level only when 1.0 mA was applied. However, in unanesthetized animals, the pattern of labeling was statistically significant at 0.2 mA. The present study demonstrates that electrical stimulation can evoke the expression of Fos-like immunoreactivity by activating nociceptors in the absence of tissue injury, and that the use of anesthetics can modulate this expression.

Effects of vagal afferent nerve stimulation on noxious heat-evoked Fos-like immunoreactivity in the rat lumbar spinal cord

Electrophysiological and behavioral studies have described modulation of nociception by vagal afferent fibers. The objectives of this study were to 1) use Fos-like immunoreactivity as a marker for neuronal activity to examine populations of neurons in the spinal cord that are activated by a noxious heat stimulus, 2) determine whether heat-evoked Fos-like immunoreactivity can be modulated by vagal afferent stimulation, and 3) determine whether vagally-mediated effect on heat-evoked Fos-like immunoreactivity can be blocked by intrathecally administered serotoninergic receptor and alpha-adrenergic receptor antagonists. Neurons demonstrating Fos-like immunoreactivity were located in the ipsilateral superficial and deep dorsal horn laminae extending from the caudal L3 through the rostral L6 region of the spinal cord. Stimulation of the right cervical vagus nerve attenuated significantly (42%) heat-evoked Fos-like immunoreactivity in the superficial laminae. The reduction in Fos-like immunoreactivity by vagal stimulation was abolished by intrathecal administration of methysergide, a nonselective serotoninergic receptor antagonist, but not by phentolamine, a nonselective alpha-adrenoceptor antagonist. These results suggest that vagal afferent modulation of spinal nociceptive transmission is mediated, at least in part, by serotonin receptors.

Effects of opioids and non-opioids on c-Fos-like immunoreactivity induced in rat lumbar spinal cord neurons by noxious heat stimulation

This study evaluated Fos-like immunoreactivity in rat lumbar spinal cord neurons following peripheral noxious heat stimulation and the modifications induced by pharmacological agents. Under urethane anaesthesia, the hindpaw was stimulated by dipping it in a regulated temperature bath at various temperatures (44-65 degrees C) and for various durations (5 s to 2 min). There was no Fos-like immunoreactivity in lumbar spinal cord neurons when the paw was stimulated at 44 degrees C for 15 s. From 46 to 52 degrees C, the number of Fos-like immunoreactivity neurons increased with increasing stimulation temperature, but was decreased at 65 degrees C as compared to 52 degrees C. At 52 degrees C, the number of Fos-like immunoreactivity neurons increased with the duration of stimulation. Fos-like immunoreactive neurons in the L4 segment were almost exclusively located in laminae I-II. On the basis of the results of the latter experiments, we chose a stimulation of 52 degrees C for 15 s to perform pharmacological investigations. The number of Fos-like immunoreactive neurons induced by the heat stimulation was significantly decreased by pretreatment with morphine (42, 64 and 75% decrease as compared to control values after 2.5, 5 and 7.5 mg/kg i.v. respectively), and these effects were blocked by naloxone. When various stimulation intensities (46-52 degrees C) were used, the effects of morphine (5 mg/kg i.v.) were most marked when the temperature was highest. In morphine-tolerant rats, morphine (5 mg/kg i.v.) was half as potent in decreasing Fos-like immunoreactivity induced by the heat stimulation than in non-tolerant rats. RB 101, a systemically active mixed inhibitor of enkephalin-metabolising enzymes, significantly decreased Fos-like immunoreactivity induced by heat stimulation (19, 29 and 48% decreases as compared to control values at 10, 20 and 40 mg/kg i.v. respectively) and these effects were blocked by naloxone. Aspirin (150 mg/kg i.v.), proacetaminophen (300 mg/kg i.v.) and tizanidine, a centrally acting myorelaxant (0.25-1 mg/kg i.v.), had no effect on the number of Fos-like immunoreactivity neurons induced by heat stimulation. The use of immunochemistry of the c-Fos protein as a pharmacological test in order to gauge antinociceptive effects at the dorsal horn level is discussed.

Distribution of c-fos expressing dorsal horn neurons after electrical stimulation of low threshold sensory fibers in the chronically injured sciatic nerve

The distribution of proto-oncogene c-Fos protein-immunoreactive cells in the spinal cord dorsal horn was studied after electrical stimulation at A alpha/A beta-fiber intensity of normal and previously injured sciatic nerves in urethane anesthetized rats. No or only occasional Fos protein-like immunoreactive cells were seen after stimulation of the normal uninjured nerve or after nerve transection without stimulation. Electrical nerve stimulation at 3, 12, and 21 days after sciatic nerve transection resulted in substantial increases in the numbers of Fos protein-like immunoreactive cell nuclei in each of Rexed's laminae I-V. Combined demonstration of Fos protein-like immunoreactivity and of glial fibrillary acidic protein-like immunoreactivity (astroglia) or OX-42 immunoreactivity (microglia), indicated that the observed Fos protein-like response was confined to neurons and not to astroglia or microglia. Combined demonstration in the spinal cord of Fos protein-like immunoreactive neurons and neurons labeled retrogradely with Fluoro-Gold from the gracile nucleus showed that some of the Fos protein-like immunoreactive neurons in Rexed's laminae III and IV contributed to the postsynaptic dorsal column pathway. The results indicate that stimulation at A alpha/A beta-fiber intensity of a previously injured nerve gives rise to an abnormally increased activation pattern of postsynaptic neurons in the dorsal horn, some of which contribute to the postsynaptic dorsal column pathway.

Chronic treatments with aspirin or acetaminophen reduce both the development of polyarthritis and Fos-like immunoreactivity in rat lumbar spinal cord

We have previously shown that during the development of adjuvant-induced arthritis (AIA), and without any peripheral stimulation, the number of Fos-like immunoreactive (Fos-LI) neurons in lumbar spinal cord increases in parallel with the clinical and behavioral signs of the disease and peaks 3 weeks after the inoculation which corresponds to the maximal stage of hyperalgesia (Abbadie and Besson 1992a). The aim of this study was to evaluate the suitability of the Fos-LI technique to gauge the effects of the two most prescribed analgesics, aspirin and acetaminophen (paracetamol), on spinal cord neurons of polyarthritic rats. The effects of the two drugs were tested on the "evoked" Fos-LI induced by peripheral mechanical noxious stimulus, as well as the effects of a chronic treatment on "basal" Fos-LI appearing during the development of polyarthritis in the absence of any intentional stimulation. We showed that: (1) Fos-LI evoked by ankle stimulation was not modified by either aspirin (150 mg/kg i.v.) or pro-acetaminophen (300 mg/kg i.v.) injection or by a 10-day chronic treatment with acetaminophen (250 or 500 mg/kg/day). (2) Despite the fact that the clinical signs of arthritis were reduced, basal Fos-LI induced by AIA disease was not changed after a 2-week chronic treatment with either aspirin (300 mg/kg/day) or acetaminophen (500 mg/kg/day) starting 3 weeks after AIA inoculation, i.e., at the maximal stage of hyperalgesia and when Fos-LI is maximal. This observation questions the suitability of Fos-LI technique to gauge the effects of mild analgesics. (3) In contrast, when the same chronic treatment was applied during the development of AIA, i.e., 1 week after inoculation, the number of Fos-LI nuclei was significantly decreased (about 50%) in aspirin- and acetaminophen-treated groups as compared to vehicle-treated groups. In parallel, the clinical signs of AIA disease were blocked by the two drug treatments. In addition, 2 weeks after the end of treatment, neither the clinical signs nor the number of Fos-LI increased again. The fact that the two drugs are able to prevent c-fos expression during development of arthritis, but not to interfere with already existing c-fos expression, suggests that for pharmacological investigations this technique should be used with caution. Thus, the potential use of Fos-LI to gauge the effects of non-steroidal antinociceptive drugs and other mild analgesics during chronic disease such as arthritis is discussed.

Intense cold noxious stimulation of the rat hindpaw induces c-fos expression in lumbar spinal cord neurons

This study evaluated Fos-like immunoreactivity in lumbar spinal cord neurons following intense cold stimulation and then the modifications induced by opioid administration. Under urethane anaesthesia, the rat's right foot was stimulated by holding it in a regulated temperature bath at 15, 10, 0, -10, -15, -17.5 or -20 degrees C. There was no or little Fos-like immunoreactivity in lumbar spinal cord neurons when the paw was at temperatures between 15 and -10 degrees C (0-5 Fos-like immunoreactive neurons/section). The threshold to induce consistent c-fos expression was -15 degrees C. From -15 to -20 degrees C, the number of Fos-like immunoreactive neurons increased with decreases in temperature. At -20 degrees C, Fos-like immunoreactive neurons were numerous in L3 and L4 segments, in laminae I-II (approximately 60 Fos-like immunoreactive neurons/section) and to a lesser extent in laminae V-VI (approximately 20). Almost no Fos-like immunoreactivity was present in laminae III-IV (< 5). At -20 degrees C, the number of Fos-like immunoreactive neurons increased with the duration of the stimulation. The number of Fos-like immunoreactive neurons induced by the cold stimulation temperatures was significantly decreased by pretreatment with 10 mg/kg s.c. morphine and moderately decreased by 5 mg/kg s.c. This effect was antagonized by the combined administration of morphine (10 mg/kg s.c.) and naloxone (2 mg/kg s.c.). Naloxone (2 mg/kg s.c.) significantly increased the number of Fos-like immunoreactive neurons induced by -20 degrees C as compared to saline-injected rats. This study showed that Fos-like immunoreactivity distribution is in good agreement with the location of neurons receiving noxious inputs and that the threshold to induce c-fos expression with cold was unexpectedly low at -15 degrees C. Taking into account, on the one hand, previous investigations using the same technique using noxious heat stimulation and, on the other hand, electrophysiological and psychophysiological studies using cold stimulation in animals and humans, our results suggest that Fos-like immunoreactivity induced by extremely cold stimulation, which seems to reproduce frostbite, may reflect activation of nociceptors due to vasoconstriction.

Induction of Fos-like protein in neurons of the medulla oblongata after electrical stimulation of the vagus nerve in anesthetized rabbit

Antibodies against the c-fos protein product Fos were used to map the first- and higher-order neurons in the rabbit medulla oblongata after electrical stimulation of the vagus nerve. Fos immunoreactivity appeared bilaterally except in the nucleus tractus solitarii. Seven areas were labeled: the nucleus tractus solitarii, the area postrema, the subnucleus lateralis caudalis magnocellularis medullar oblongata, the lateral reticular nucleus, the ambiguus nucleus, the dorsal part of the spinal trigeminal nucleus, the nucleus reticularis lateralis, the lateral border of the external cuneatus nucleus, the medial part of the inferior olivary nucleus (subnucleus beta). The last two areas have never been visualized with conventional tracing techniques and may represent higher-order neurons connected to visceral vagal pathways. No labeling was observed in the nodose ganglion.