C-fos Induction in the Spinal Cord after Peripheral Nerve Lesion

JI017 Attenuates Oxaliplatin-Induced Cold Allodynia via Spinal TRPV1 and Astrocytes Inhibition in Mice

Oxaliplatin, a well-known chemotherapeutic agent, can induce severe neuropathic pain, which can seriously decrease the quality of life of patients. JI017 is an herb mixture composed of Aconitum carmichaelii, Angelica gigas, and Zingiber officinale. Its anti-tumor effect has been reported; however, the efficacy of JI017 against oxaliplatin-induced allodynia has never been explored. Single oxaliplatin injection [6 mg/kg, intraperitoneal, (i.p.)] induced both cold and mechanical allodynia, and oral administration of JI017 (500 mg/kg) alleviated cold but not mechanical allodynia in mice. Real-time polymerase chain reaction (PCR) analysis demonstrated that the upregulation of mRNA of spinal transient receptor potential vanilloid 1 (TRPV1) and astrocytes following oxaliplatin injection was downregulated after JI017 treatment. Moreover, TRPV1 expression and the activation of astrocytes were intensely increased in the superficial area of the spinal dorsal horn after oxaliplatin treatment, whereas JI017 suppressed both. The administration of TRPV1 antagonist [capsazepine, intrathecal (i.t.), 10 μg] attenuated the activation of astrocytes in the dorsal horn, demonstrating that the functions of spinal TRPV1 and astrocytes are closely related in oxaliplatin-induced neuropathic pain. Altogether, these results suggest that JI017 may be a potent candidate for the management of oxaliplatin-induced neuropathy as it decreases pain, spinal TRPV1, and astrocyte activation.

Progression of myocardial ischemia leads to unique changes in immediate-early gene expression in the spinal cord dorsal horn

The pathological consequences of ischemic heart disease involve signaling through the autonomic nervous system. Although early activation may serve to maintain hemodynamic stability, persistent aberrant sympathoexcitation contributes to the development of lethal arrhythmias and heart failure. We hypothesized that as the myocardium reacts and remodels to ischemic injury over time, there is an analogous sequence of gene expression changes in the thoracic spinal cord dorsal horn, the processing center for incoming afferent fibers from the heart to the central nervous system. Acute and chronic myocardial ischemia (MI) was induced in a large animal model of Yorkshire pigs, and the thoracic dorsal horn of treated pigs, along with control nonischemic pigs, was harvested for transcriptome analysis. We identified 32 differentially expressed genes between healthy and acute ischemia cohorts and 46 differentially expressed genes between healthy and chronic ischemia cohorts. The canonical immediate-early gene c-fos was upregulated after acute MI, along with fosB, dual specificity phosphatase 1 and 2 ( dusp1 and dusp2), and early growth response 2 (egr2). After chronic MI, there was a persistent yet unique activation of immediate-early genes, including fosB, nuclear receptor subfamily 4 group A members 1-3 ( nr4a1, nr4a2, and nr4a3), egr3, and TNF-α-induced protein 3 ( tnfaip3). In addition, differentially expressed genes from the chronic MI signature were enriched in pathways linked to apoptosis, immune regulation, and the stress response. These findings support a dynamic progression of gene expression changes in the dorsal horn with maturation of myocardial injury, and they may explain how early adaptive autonomic nervous system responses can maintain hemodynamic stability, whereas prolonged maladaptive signals can predispose patients to arrhythmias and heart failure. NEW & NOTEWORTHY Activation of the autonomic nervous system after myocardial injury can provide early cardiovascular support or prolonged aberrant sympathoexcitation. The later response can lead to lethal arrhythmias and heart failure. This study provides evidence of ongoing changes in the gene expression signature of the spinal cord dorsal horn as myocardial injury progresses over time. These changes could help explain how an adaptive nervous system response can become maladaptive over time.

Review : The Immediate-Early Gene Response and Neuronal Death and Regeneration

The induction of the cellular immediate-early gene (cIEG) response is the earliest known transcriptional con sequence of neuronal excitation. It is believed that the products of the cIEGs (such as c- fos and c- jun) serve to bring about persistent changes in neuronal phenotype by altering gene expression and modifying signal transduction pathways. This has led, on the one hand, to the burgeoning use of Fos immunohistochemistry as a surrogate form of neuronal activity mapping and, on the other hand, to a quest for the biochemical functions of individual cIEG products in the nervous system. Recent studies of neuronal death and regen eration have contributed substantially to our vision of the molecular and biological characteristics of the cIEG response. Indeed, they have challenged some of its long-held tenets. Therefore, we will use these results to illustrate our most contemporary view of the cIEG response in the nervous system. The Neuroscientist 1:68- 75,1995

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Changes in phosphorylation of ERK and Fos expression in dorsal horn neurons following noxious stimulation in a rat model of neuritis of the nerve root

Mechanical compression and chemical inflammation of the spinal nerve root are considered major sensory pathologies secondary to a lumbar disc herniation. In order to elucidate the dorsal horn responsiveness to noxious stimulation to the peripheral tissue in the neuritis model of the nerve root, we examined extracellular signal-regulated kinase (ERK) phosphorylation and Fos expression in spinal cord dorsal horn neurons. Male Sprague-Dawley rats received hemilaminectomies and the implantation of disc tissue that was obtained from coccygeal intervertebral discs. Three or 7 days after surgery, rats were perfused after receiving noxious mechanical stimulation of the plantar surface of the hind paw using a hemoclip, and the L4/5 spinal cord was processed for immunohistochemistry with antibodies for phospho-ERK and Fos. The number of Fos-immunoreactive (Fos-LI) neurons and phospho-ERK-immunoreactive (phospho-ERK-LI) neurons in the neuritis group after the noxious stimulation significantly increased compared to the sham-treated group at 3 and 7 days after surgery. The change in number of phospho-ERK-LI and Fos-LI neurons occurred mainly in the superficial dorsal horn. The number of Fos-LI neurons observed when the MEK inhibitor, U0126, was administered was significantly suppressed compared to the DMSO- (vehicle control) administered group. The increase in ERK phosphorylation and Fos expression in the spinal cord dorsal horn neurons indicates that responses/activation by the noxious stimulation applied to the periphery were elevated in spinal cord neurons in this neuritis model of the lumbar nerve root. Moreover, the increase in the Fos expression in the spinal cord dorsal horn may have been the result of the activation of the MAP kinase cascade.

Hypodynamia--hypokinesia induced variations in expression of fos protein in structures related to somatosensory system in the rat

There have been many reports describing modifications of the sensory and motor cortex following various types of disuse. Hypodynamia--hypokinesia is characterized by the absence of weight-bearing and by a decrease in motor activity. We have shown a reorganization of the cortical cartography after hypodynamia--hypokinesia. In order to give an anatomical account for this cortical plasticity, we set out to determine whether cerebral and spinal structures exhibited variations of their neuronal activation. For this purpose, immunocytochemical detection of Fos protein was performed in the rat brain and spinal cord. Following stimulation of the sciatic nerve, Fos protein was detected in the primary and secondary somatosensory cortex in control rats and in rats submitted to an episode of 14 days of hypodynamia--hypokinesia. Results showed that the stimulation of the sciatic nerve induced an increase in the number of Fos-immunoreactive neurons in all these structures. Moreover, after hypodynamia--hypokinesia, the number of Fos-immunoreactive neurons was increased in the primary and secondary somatosensory cortex and in the spinal cord. These results provide evidence for a higher activation of cortical cells after hypodynamia--hypokinesia in comparison to controls. These data support the hypothesis that hypodynamia--hypokinesia contributes to the development of functional plasticity.

The effect of neonatal capsaicin on the c-Fos-like immunoreactivity induced in subnucleus oralis neurons by noxious intraoral stimulation

The noxious stimulus-dependent induction of c-Fos-like immunoreactivity (Fos-LI) in neurons in the subnucleus oralis and the medullary dorsal horn (MDH) was significantly suppressed by the selective destruction of unmyelinated primary neurons. The induction of Fos-LI by topical capsaicin application to the lingual mucosal stimulation was almost completely suppressed by neonatal capsaicin treatment. Fos-LI induction by the tooth pulp stimulation and by formalin injection to the lingual mucosa were only partially reduced. These results provide an evidence that the noxious signals from the intraoral structures are transmitted by both unmyelinated and myelinated nociceptors to the subnucleus oralis as well as the MDH.

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.

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

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

Ketamine suppresses c-fos expression in dorsal horn neurons after acute constrictive sciatic nerve injury in the rat

Fos immunoreactivity within the spinal cord in a model of neuropathic pain was studied. Dorsal horn neurons in laminae I and II exhibited selective expression within the tibial, peroneal and posterior cutaneous nerve territories which, in turn, was suppressed during ketamine but not halothane anesthesia. Fos immunoreactive neurons have a unique response pattern to neuropathic pain which is sensitive to ketamine.

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

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

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

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

c-fos induction in the subnucleus oralis following trigeminal nerve stimulation

Neurons with c-Fos protein-like immunoreactivity (fos-neurons) were examined in the rostral parts of the brainstem sensory trigeminal nuclear complex following intense electrical stimulation of the trigeminal nerves and noxious mechanical stimulation of the trigeminal receptive fields. Stimulation of all the examined nerves and receptive fields induced some fos-neurons at the medial edge of the subnucleus interpolaris but not in the principal sensory trigeminal nucleus. Stimulation of the primary neurons innervating the intraoral structures but not facial skin induced fos-neurons in the ipsilateral subnucleus oralis. These oralis fos-neurons were located in the dorsomedial nucleus that contained calcitonin gene-related peptide-like immunoreactivity. The oralis fos-neurons are considered to be involved in the processing of intraoral nociceptive signals.

Exaggerated C-fiber activation prevents peripheral nerve injury-induced hyperinducibility of c-Fos in partially deafferented spinal dorsal horn

Dorsal horn neurons chronically deafferented by peripheral nerve injuries acquire hypersensitivity to noxious input from outside the original receptive field. This study examines the effect of electrical nerve stimulation at the time of injury on such injury-induced hypersensitivity. The medial 3/8 of the dorsal horn laminae I/II around the junction of 4th and 5th lumbar segments (the tibial territory) was deafferented by transection of the ipsilateral tibial nerve in rats. At 2 days or 3 weeks postinjury, the hindpaw was injected with formalin to induce c-fos. At 2 days, neurons with induced c-Fos protein-like immunoreactivity (fos-neurons) were largely confined in the lateral 5/8 of laminae I/II (the peroneal and hip, thus P and H territory). At 3 weeks, fos-neurons significantly increased in the deafferented tibial territory. A similar increase was also noted in the P and H territory. Thus the dorsal horn neurons exhibited c-fos hyperinducibility, an indication of hypersensitivity. Electrical stimulation with a train of 150 shocks (10 V, 2 ms) of the proximal nerve stump immediately after transection prevented the c-fos hyperinducibility. The effect was greater with the stimulation frequency of 0.5 Hz than 0.1 Hz or 10 Hz. The stimulation had no effect on the c-fos inducibility at 2 days postinjury.

Neuropeptide changes persist in spinal cord despite resolving hyperalgesia in a rat model of mononeuropathy

We have previously described the changes in spinal cord neuropeptides in the unilateral sciatic chronic constriction injury (CCI) model of Bennett and Xie [Pain, 33 (1988) 87-108] at 28 days, a time of maximum mechanical hyperalgesia. In this study we examine the same model 100-120 days post injury by which time resolution of the hyperalgesia and peripheral nerve injury has occurred according to previous studies. Rats underwent either CCI of the sciatic nerve (n = 12) or else sham operation (n = 8) which involved exposure but no ligation of the nerve. Mechanical hyperalgesia was assessed with a Ugo-Basile analgesymeter and immunohistochemistry performed on the spinal cord sections of the animals and quantified using a confocal microscope. At this late time point CCI rats were no longer significantly mechanically hyperalgesic compared to the sham animals (P > or = 0.09). However, examination of the lumbar spinal cord revealed the following changes. (i) The neuropeptides substance P (SP) (P < 0.0001) and galanin (P < 0.003) both showed decreases of about 30% ipsilaterally in immunoreactivity in laminae 1 and 2 of the dorsal horn compared to the sham operated animals. (ii) Calcitonin gene-related peptide (CGRP) and neuropeptide Y (NPY) in laminae 1 and 2 showed no significant changes compared to sham animals. (iii) NPY levels in laminae 3 and 4 of the spinal cord showed a 15% increase in immunoreactivity compared to sham animals (P = 0.008). These results indicate that changes in neuronal markers in the spinal cord can persist after apparent resolution of a peripheral nerve injury. We suggest that these changes may form a substrate for subsequent development of abnormal pain states.

Changes in neuronal markers in a mononeuropathic rat model relationship between neuropeptide Y, pre-emptive drug treatment and long-term mechanical hyperalgesia

Using the chronic constriction model (CCI) of Bennett and Xie (1988), changes in the lumbar spinal cord in neuropeptides and lectin IB4 were examined at 28 days post-nerve constriction and were compared with the degree of mechanical hyperalgesia. Animals following nerve ligation were significantly more hyperalgesic than sham-operated animals (P < 0.0001). Lectin IB4, a marker of primary afferent C fibres, showed a qualitative decrease in staining intensity in laminae 1-2 with ligation compared with both the unoperated contralateral side and with sham animals. Using fluorescent immunohistochemistry to quantify changes in neuropeptides in the dorsal horn we found that substance P showed significant decreases with ligation compared to sham operation (P < 0.002). CGRP and galanin showed no significant changes in laminae 1-2 compared to sham-operated animals. Neuropeptide Y (NPY) showed no significant changes in intensity in laminae 1-2; however, in laminae 3-4 there was a significant increase with nerve ligation compared to sham (P < 0.005). We examined how pre-emptive drug treatment affected these neuronal markers at 28 days. We used (1) clonidine, an alpha 2-adrenoreceptor agonist (1 mg/kg, i.p.), (2) morphine, a mu-opioid agonist (5 mg/kg, i.p.) or (3) MK-801, an N-methyl-D-aspartate (NMDA) receptor antagonist (0.3 mg/kg, s.c.) administered 30 min prior and 6 h following nerve ligation or sham-operation. Hyperalgesia in the ligated group at 28 days was suppressed by treatment with pre-emptive clonidine (P = 0.003) or MK-801 (P = 0.003) but not morphine. With the exception of NPY there was no effect of pre-emptive drug treatment on any neuronal marker examined. Pre-emptive MK-801 reduced the magnitude of the increase in NPY in laminae 3-4 in the ligated group (P < 0.005) and clonidine showed a similar trend but this did not reach significance. Morphine had no effect on NPY staining. There was a significant correlation between the increase in NPY staining in laminae 3-4 and the degree of hyperalgesia (r = 0.6, P < 0.001). These results suggest that the increased NPY expression in laminae 3-4 of the spinal cord (the territory of the myelinated sensory input) may be crucial to the development of hyperalgesia in this model.

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.

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.

c-Fos induction in the rat spinal dorsal horn partially deafferented by dorsal rhizotomy

The 4th and 5th segments of the lumbar (the L4 and L5) dorsal horn receive primary input from the sciatic receptive fields through the L4 and L5 dorsal roots. Noxious stimulation of the hindpaw with formalin induces c-Fos in neurons in superficial laminae (I and II) of these dorsal horn segments. Rhizotomy of the L5 dorsal root 2 days before stimulation resulted in a marked reduction in the number of neurons with c-Fos protein-like immunoreactivity (fos-neurons). At 3 weeks after the L5 rhizotomy, the number of fos-neurons in laminae I and II significantly increased compared to that at 2 days post-rhizotomy. This result indicates that chronic partial deafferentation by dorsal rhizotomy increases responsiveness of superficial dorsal horn neurons to spared primary input.

The resolution of neuropathic hyperalgesia following motor and sensory functional recovery in sciatic axonotmetic mononeuropathies

Nerve lesions producing extensive axonal loss can induce painful hyperalgesic states in man. The affect of axonal regeneration and end-organ reinnervation on hyperalgesia and pain is controversial. This study used two axonotmetic models, the sciatic crush injury (CI) and the sciatic chronic constrictive injury (CCI), to investigate the affects of nerve regeneration and reinnervation on hyperalgesia and presumed painful behavior in rats. The sciatic CI resulted in a transient loss of both sciatic motor function and the withdrawal response to pinch and heat in the sciatic distribution. Extensive recovery of motor function, pinch and heat response occurred over days 23-38 post-crush injury. This temporally corresponded with a plateau in the hindpaw autotomy score and a resolution of the saphenous-mediated pressure and heat hyperalgesia (adjacent neuropathic hyperalgesia; ANH) which developed over the medial dorsum of the hindpaw following the sciatic CI. In contrast, with sciatic transection and distal stump excision, no motor recovery occurs, large areas of the hindpaw remain unresponsive to heat and pinch, and the saphenous mediated ANH fails to resolve over a period of 3 months. When sciatic CI was compared to contralateral sciatic transection within the same rat, the bilateral saphenous-mediated pressure and heat thresholds were initially identical, but by 23-27 days post-crush, the crush side thresholds became hypoalgesic relative to the section side. This demonstrates an attenuation of the crush-induced ANH which temporally corresponds to the recovery of motor and sensory function. When the sciatic nerve was proximally crushed and distally transected (3 cm below the crush site), the saphenous-mediated pressure and heat threshold changes were identical (over 6 weeks of serial testing) to those produced by a contralateral sciatic transection within the same rat. This indicates that the microenvironments surrounding the regenerating axon tips did not differentially affect the development of ANH following sciatic CI or transection. The sciatic CCI resulted in a transient loss of hindpaw motor function without the loss of pinch or heat withdrawal responses in the sciatic distribution. Motor function recovery occurred primarily over days 23-59 post-ligature. During this prolonged period of motor function recovery there was a resolution of the sciatic-mediated plantar surface heat hyperalgesia and the saphenous-mediated heat ANH. The above data support the hypothesis that the successful regeneration of distal axons after axonotmetic lesions can initiate the resolution of neuropathic hyperalgesia.

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.

Changes in c-fos induction in dorsal horn neurons by hindpaw formalin stimulation following tibial neurotomy

The hindpaw was partially denervated by the tibial nerve transection in adult rats. At post-transection intervals varying from 2 to 168 days, the hindpaw was stimulated bilaterally by subcutaneous injection of formalin. The excitability of dorsal horn neurons was expressed as the percentage ratio of the number of formalin-induced c-fos protein-like immunoreactive neurons (fos-neurons) on the neurotomized (experimental) side to that on the un-neurotomized (control) side. At 2 days post-injury, a marked reduction in the number of fos-neurons was noted in laminae I-VII of the lumbar spinal cord. Among these, reduction was greatest in the medial 3/8 of laminae I and II (terminal field of the tibial nerve, i.e. tibial territory), and smallest in the lateral 5/8 of the same laminae (the peroneal/hip territory). The low level of c-fos induction remained unchanged for 7 days. At 14 days, the excitability of neurons in all laminae showed a marked increase compared to the post-injury days 2 and 3 combined. Thereafter, the increased level of excitability in the tibial territory was maintained throughout the post-injury period examined in this study. On the other hand, a statistically significant increase in excitability in the peroneal/hip territory was only seen between 14 and 28 days and the excitability almost returned to the baseline (days 2 and 3 post-transection combined) level at 42 days. Although deeper laminae (III-VII) contained much less formalin-induced fos-neurons, they also exhibited post-injury excitability changes with a temporal pattern similar to that of the peroneal/hip territory of laminae I and II.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

A loose ligature-induced mononeuropathy produces hyperalgesias mediated by both the injured sciatic nerve and the adjacent saphenous nerve

An experimental mononeuropathy was produced in rats by unilateral loose ligation of the sciatic nerve, with a contralateral sham surgery. Repeated bilateral sensory testing was performed over time to investigate any threshold differences between the loose ligature side and the contralateral control side. A prolonged reduction in the mean withdrawal threshold to heat was observed on the loose ligature side over the medial dorsum of the hindpaw (MDH), and over the plantar surface of the hindpaw (PSH). Following loose ligature there was also a significant reduction in the von Frey fiber withdrawal thresholds over the lateral dorsum of the hindpaw (LDH) and the medial dorsum of the hindpaw (MDH). In a second experiment the saphenous nerves were transected bilaterally in rats who had an earlier unilateral loose sciatic ligation. The heat hyperalgesia that had been previously observed over the MDH was abolished, and no withdrawal response could be elicited with the largest von Frey fiber over the MDH, indicating that the MDH hyperalgesia induced by sciatic loose ligature was mediated by the saphenous nerve. This adjacent neuropathic hyperalgesia (ANH) resembles the saphenous mediated ANH observed over the MDH following sciatic transection, follows the same temporal course of onset, and may share a similar pathogenesis. The heat threshold over the PSH and the pressure threshold over the LDH were unaffected by the bilateral saphenous transection, demonstrating that following loose sciatic ligation both of these thresholds are mediated by the sciatic nerve.

c-Fos expression by dorsal horn neurons chronically deafferented by peripheral nerve section in response to spared, somatotopically inappropriate nociceptive primary input

Subcutaneous formalin injection into the hindpaw of rats induces c-Fos expression in neurons in the ipsilateral spinal cord dorsal horn. In laminae I and II of the dorsal horn at the junction of 4th and 5th segments of the lumbar spinal cord, neurons exhibiting c-Fos protein-like immunoreactivity (Fos-LI) are concentrated in the medial 3/4 that correspond to the terminal field of primary neurons innervating the sciatic nerve. Subacute tibial nerve section 24 h before formalin stimulation caused almost complete elimination of neurons with the formalin-induced Fos-LI in the medial 1/2 (tibial territory) of the above sciatic territory of the dorsal horn. Following a longer survival period (chronic tibial nerve section of 21 days standing), neurons with the formalin-induced Fos-LI re-appeared in the tibial territory. In addition, the number of neurons with the formalin-induced Fos-LI increased in the medial part of the peroneal territory (the lateral 1/2 of the sciatic territory). The results indicate that the activation of c-Fos expression in that part of dorsal horn that has been chronically deafferented by the tibial nerve section is taken over by the spared, but somatotopically inappropriate primary nociceptors. Furthermore, dorsal horn neurons outside but near the deafferented tibial nerve's territory exhibit hypersensitivity to c-Fos expression evoked by intact, somatotopically appropriate primary nociceptive input.

Peripheral and central contributions to the persistent expression of spinal cord fos-like immunoreactivity produced by sciatic nerve transection in the rat

Previous studies have demonstrated that noxious stimuli, intense enough to produce tissue injury, evoke a transient expression of the Fos protein product of the c-fos proto-oncogene in neurons, in regions of the spinal cord that contribute to the transmission of nociceptive messages in the rat. Since there is evidence that increases in fos-like immunoreactivity reflect increases in neuronal activity, it has thus been possible to identify populations of neurons that are activated in response to tissue injury. In this study we used immunocytochemical localization of fos-like immunoreactive (FLI) neurons to map the patterns of neuronal activity in the spinal cord at different times after peripheral nerve injury in the rat. Sciatic nerve transection induced a persistent (at least 1 month) elevation in the number of FLI neurons, predominantly in laminae 1, 2, 5, 6 and 7 of the ipsilateral lumbar enlargement of the spinal cord. In the L5 segment, the expression of fos-like immunoreactivity in the superficial dorsal horn (laminae 1 and 2) fluctuated, with peaks of Fos expression at 2 h, 2 days and 2 weeks after nerve transection. Furthermore, by 2 weeks after nerve injury, the distribution of labelled neurons in the superficial laminae of the dorsal horn shifted, with the most densely labelled cells now located in the central portion of the superficial dorsal horn. In contrast, the pattern of labelled neurons in laminae 5, 6 and 7 was relatively constant over the 4-week study period. Local anesthetic block of the sciatic nerve significantly decreased the number of FLI neurons when it was administered at either 2 days or 2 weeks post nerve injury. At 2 days, injection of the local anesthetic subcutaneously in the dorsum of neck, to control for a systemic action, also reduced expression of FLI in laminae 1 and 2; at 2 weeks, the systemic injection of the local anesthetic reduced expression of FLI throughout the gray matter of the spinal cord. These results demonstrate that peripheral nerve injury, in contrast to tissue injury, induces a prolonged increase in Fos expression in neurons predominantly in those regions of the spinal cord that are associated with the transmission of nociceptive messages. This pattern of fos-like immunoreactivity is probably the result of persistent neuronal activity in the spinal cord. The increased 'activity' in the spinal cord appears to be maintained both by abnormal activity in the injured peripheral nerve as well as by reorganization of circuits within the spinal cord secondary to the nerve injury.

c-fos expression in gracilothalamic tract neurons after electrical stimulation of the injured sciatic nerve in the adult rat

The number of c-fos protein-like immunoreactive (Fos-LI) cells in the gracile nucleus was determined after electrical stimulation at A alpha/A beta-fiber strength of the normal and of the previously injured sciatic nerve in adult rats. No Fos-LI cells were seen after electrical stimulation of the noninjured sciatic nerve, or after sciatic nerve injury without electrical stimulation. However, stimulation 21 days after sciatic nerve transection resulted in numerous Fos-LI cells in the ipsilateral gracile nucleus. Combined Fos immunocytochemistry and retrograde labeling from the thalamus showed that the majority (76%; range = 70-80%) of the cells in the gracile nucleus that expressed Fos-LI after nerve injury projected to the thalamus. The results indicate that morphological, biochemical, and physiological alterations in primary sensory central endings and second-order neurons, which have earlier been demonstrated in the dorsal column nuclei after peripheral nerve injury, are accompanied by changes in the c-fos gene activation pattern after stimulation of the injured sciatic nerve. A substantial number of the c-fos-expressing neurons project to the thalamus.

Changing pattern of c-FOS expression in spinal cord neurons after electrical stimulation of the chronically injured sciatic nerve in the rat

Immunocytochemical technique was used to study the distribution of c-FOS protein immunoreactive cells in the spinal cord and gracile nuclei 2 h after electrical stimulation of the sciatic nerve in ketamine/xylazine/acepromazine-anesthetized adult rats. Quantitative examination of the c-fos-labeled cells in the spinal cord laminae was made in unoperated and sham operated controls, after sciatic nerve transection without electrical stimulation, and after electrical stimulation at C-fiber or A alpha/beta-fiber intensity, both in normal animals and at various survival times after chronic sciatic nerve injury (transection and ligation) or crush. Unoperated animals showed very few c-fos-labeled cells, and sham operated controls showed labeled cells located mainly outside the sciatic nerve projection territory. A small increase in number of c-fos protein positive cells was seen after sciatic nerve transection without electrical stimulation. Stimulation of the normal sciatic nerve at C-fiber intensity resulted in c-fos protein-positive cells within the sciatic projection territory in the ipsilateral dorsal horn. Labeled cells were seen in all spinal cord laminae except lamina IX, with the vast majority in lamina I and outer lamina II. No labeled cells were seen in the gracile nucleus. Stimulation at A alpha/beta fiber intensity resulted in no or only a very small number of c-fos-positive neurons. Electrical stimulation of the injured sciatic nerve at C-fiber intensity, using the uninjured contralateral side as control, resulted in significant decreases in c-fos-immunoreactive cells in lamina I plus the outer portion of lamina II at 12 and 39 days survival after injury. A non-significant decrease was seen in these laminae also after 21 days. Significant increases were seen in laminae III and IV at 21 days. Decreases in laminae V, VI and more ventral laminae were significant at 21 and 39 days after injury. At longer survival times, the difference between the normal and injured side seen weeks after injury tended to disappear. Stimulation at A alpha/beta fiber intensity 21 days after injury resulted in increases in the numbers of labeled cells in ipsilateral laminae II, III and IV and in the gracile nucleus. Sciatic nerve stimulation after crush injury resulted in more variable side differences, with tendencies for the same alterations as those noted after chronic transection-ligation.(ABSTRACT TRUNCATED AT 400 WORDS)