Fos-like immunoreactivity in the rat superficial dorsal horn induced by formalin injection in the forepaw: effects of dorsal rhizotomies

Functional and anatomical analyses of active spinal circuits in a mouse model of chronic pain

ABSTRACT

Decades of efforts in elucidating pain mechanisms, including pharmacological, neuroanatomical, and physiological studies have provided insights into how nociceptive information transmits from the periphery to the brain and the locations receiving nociceptive signals. However, little is known about which specific stimulus-dependent activated neurons, amongst heterogeneous neural environments, discriminatively evoke the cognate pain behavior. We here shed light on the population of neurons in the spinal cord activated by a painful stimulus to identify chronic pain-dependent activated neuronal subsets using Fos2A-iCreER (TRAP2) mice. We have found a large number of neurons activated by a normally nonpainful stimulus in the spinal cord of spinal nerve-ligated mice, compared with sham. Neuronal activation was observed in laminae I and II outer under heat hyperalgesia. A large number of neurons in laminae II inner were activated in both mechanical allodynia and heat hyperalgesia conditions, while mechanical allodynia tends to be the only stimulus that activates cells at lamina II inner dorsal region. Neuroanatomical analyses using spinal cell markers identified a large number of spinal inhibitory neurons that are recruited by both mechanical allodynia and heat hyperalgesia. Of interest, spinal neurons expressing calretinin, calbindin, and parvalbumin were activated differently with distinct pain modalities (ie, mechanical allodynia vs heat hyperalgesia). Chemogenetic inhibition of those activated neurons significantly and specifically reduced the response to the pain stimulus associated with the stimulus modality originally given to the animals. These findings support the idea that spinal neuronal ensembles underlying nociceptive transmission undergo dynamic changes to regulate selective pain responses.

Persistent restoration of sensory function by immediate or delayed systemic artemin after dorsal root injury

Dorsal root injury results in substantial and often irreversible loss of sensory functions as a result of the limited regenerative capacity of sensory axons and the inhibitory barriers that prevent both axonal entry into and regeneration in the spinal cord. Here, we describe previously unknown effects of the growth factor artemin after crush injury of the dorsal spinal nerve roots in rats. Artemin not only promoted re-entry of multiple classes of sensory fibers into the spinal cord and re-establishment of synaptic function and simple behavior, but it also, surprisingly, promoted the recovery of complex behavior. These effects occurred after a 2-week schedule of intermittent, systemic administration of artemin and persisted for at least 6 months following treatment, suggesting a substantial translational advantage. Systemic artemin administration produced essentially complete and persistent restoration of nociceptive and sensorimotor functions, and could represent a promising therapy that may effectively promote sensory neuronal regeneration and functional recovery after injury.

Diabetes-induced chemogenic hypoalgesia is paralleled by attenuated stimulus-induced fos expression in the spinal cord of diabetic mice

Chronic hyperglycemia in diabetes induces abnormal nerve pathologies, resulting in diabetic neuropathy (DN). Sensory symptoms of DN can manifest as positive (painful), negative (insensate), or both. Streptozotocin (STZ)-induced diabetic C57Bl/6 mice have reduced cutaneous innervation and display reduced behavioral responses to noxious stimuli, reflecting the insensate aspect of the human syndrome. Current studies were undertaken to determine whether the diabetes-induced deficits in pain responses are reflected by changes in spinal activation in this model of DN. Nocifensive responses of nondiabetic and diabetic mice to formalin injection were measured 1, 3, 5, and 7 weeks after STZ, and at each time point formalin-induced spinal Fos expression was quantified. Responses of diabetic mice were significantly reduced during the second phase of the formalin test beginning 3 weeks after STZ and during Phase 1 beginning 5 weeks after STZ. Consistent with the behavioral responses, the number of Fos-positive cells in the dorsal horn of diabetic animals was significantly reduced beginning 3 weeks after STZ and continuing 5 and 7 weeks after STZ. The deficits at 5 weeks after STZ were restored by 2-week treatments with insulin or neurotrophins. These results demonstrate that the reduced sensation occurring from progressive peripheral axon loss results in functional deficits in spinal cord activation.

PERSPECTIVE

The reduced expression of the immediate early gene Fos as an indicator of pain transmission supports the diabetes-induced loss of sensation in this Type 1 model of diabetes. This murine model may be better suited to understanding the insensate symptoms of diabetic patients in the absence of chronic pain.

Intrathecal grafting of porcine chromaffin cells reduces formalin-evoked c-Fos expression in the rat spinal cord

Chromaffin cells from the adrenal gland secrete a combination of neuroactive compounds including catecholamines, opioid peptides, and growth factors that have strong analgesic effects, especially when administered intrathecally. Preclinical studies of intrathecal implantation with xenogeneic bovine chromaffin cells in rats have provided conflicting data with regard to analgesic effects, and recent concern over risk of prion transmission has precluded their use in human clinical trials. We previously developed a new, safer source of adult adrenal chromaffin cells of porcine origin and demonstrated an in vivo antinociceptive effect in the formalin test, a rodent model of tonic pain. The goal of the present study was to confirm porcine chromaffin cell analgesic effects at the molecular level by evaluating neural activity as reflected by spinal cord c-Fos protein expression. To this end, the expression of c-Fos in response to intraplantar formalin injection was evaluated in animals following intrathecal grafting of 10(6) porcine or bovine chromaffin cells. For the two species, adrenal chromaffin cells significantly reduced the tonic phases of the formalin response. Similarly, c-Fos-like immunoreactive neurons were markedly reduced in the dorsal horns of animals that had received injections of xenogeneic chromaffin cells. This reduction was observed in both the superficial (I-II) and deep (V-VI) lamina of the dorsal horn. The present study demonstrates that both xenogeneic porcine and bovine chromaffin cells transplanted into the spinal subarachnoid space of the rat can suppress formalin-evoked c-Fos expression equally, in parallel with suppression of nociceptive behaviors in the tonic phase of the test. These findings confirm previous reports that adrenal chromaffin cells may produce antinociception by inhibiting activation of nociceptive neurons in the spinal dorsal horn. Taken together these results support the concept that porcine chromaffin cells may offer an alternative xenogeneic cell source for transplants delivering pain-reducing neuroactive substances.

The contribution of autophosphorylated alpha-calcium-calmodulin kinase II to injury-induced persistent pain

Increases in neuronal activity in response to tissue or nerve injury can lead to prolonged functional changes in the spinal cord resulting in an enhancement/sensitization of nociceptive processing. To assess the contribution of alpha-calcium-calmodulin kinase II (alpha-CaMKII) to injury-induced inflammation and pain, we evaluated nociceptive responses in mice that carry a point mutation in the alpha-CaMKII gene at position 286 (threonine to alanine). The mutated protein is unable to autophosphorylate and thus cannot function independently of calcium and calmodulin. Responses to acute noxious stimuli did not differ between alpha-CaMKII T286A mutant and wild type mice. However, the ongoing pain produced by formalin injury was significantly reduced in the mutant mice, as was formalin-evoked spinal Fos-immunoreactivity. In contrast, the decreased mechanical and thermal thresholds associated with nerve injury, Complete Freund's Adjuvant-induced inflammation or formalin-evoked tissue injury were manifest equally in wild-type and mutant mice. Double-labeling immunofluorescence studies revealed that in the mouse alpha-CaMKII is expressed in the superficial dorsal horn as well as in a population of small diameter primary afferent neurons. In summary, our results suggest that alpha-CaMKII, perhaps secondary to an N-methyl-D-aspartate-mediated calcium increase in postsynaptic dorsal horn nociresponsive neurons, is a critical contributor to the spontaneous/ongoing component of tissue-injury evoked persistent pain.

Genetic evidence for the correlation of deep dorsal horn Fos protein immunoreactivity with tonic formalin pain behavior

The formalin test is commonly used as a model of persistent pain. Besides producing pain behavior, hind paw formalin injection induces the expression of the immediate-early gene, c-fos. A current controversy is whether noxious stimulus-induced Fos protein immunoreactivity can be considered a proxy (biomarker) of nociception in the spinal cord. We investigated this issue by exploiting our recent demonstration of genotype-dependent behavioral differences in response to formalin injection among inbred mouse strains. Accordingly, 6 inbred and 2 outbred strains were administered formalin (5% in 25 microL) into the ventral hind paw, monitored for licking behavior, and then sacrificed at 90 minutes after injection for Fos protein immunocytochemistry. Significant strain differences were observed in both licking behavior and Fos counts in superficial and deep laminae. We observed a significant correlation among strains between licking behavior in the late phase (10 to 60 minutes) of the formalin test and Fos expression in laminae V-VI (but not laminae I-II) of the dorsal horn (r = 0.94). These findings reinforce the use of the Fos technique to study the neuronal processing underlying pain but suggest that Fos labeling reliably reflects tonic pain behavior only in neurons located in the neck of the dorsal horn in mice.

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

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

Role of thalamic phospholipase C[beta]4 mediated by metabotropic glutamate receptor type 1 in inflammatory pain

Phospholipase C (PLC) beta4, one of the four isoforms of PLCbetas, is the sole isoform expressed in the mouse ventral posterolateral thalamic nucleus (VPL), a key station in pain processing. The mouse thalamus also has been shown to express a high level of metabotropic glutamate receptor type 1 (mGluR1), which stimulates PLCbetas through activation of Galphaq/11 protein. It is therefore expected that the thalamic mGluR1-PLCbeta4 cascade may play a functional role in nociceptive transmission. To test this hypothesis, we first studied behavioral responses to various nociceptive stimuli in PLCbeta4 knock-out mice. We performed the formalin test and found no difference in the pain behavior in the first phase of the formalin test, which is attributed to acute nociception, between PLCbeta4 knock-out and wild-type mice. Consistent with this result, acute pain responses in the hot plate and tail flick tests were also unaffected in the PLCbeta4 knock-out mice. However, the nociceptive behavior in the second phase of the formalin test, resulting from the tissue inflammation, was attenuated in PLCbeta4 knock-out mice. In the dorsal horn of the spinal cord where PLCbeta1 and PLCbeta4 mRNAs are expressed, no difference was found between the wild-type and knock-out mice in the number of Fos-like immunoreactive neurons, which represent neuronal activity in the second phase in the formalin test. Thus, it is unlikely that spinal PLCbeta4 is involved in the formalin-induced inflammatory pain. Next, we found that pretreatment with PLC inhibitors, mGluR1 antagonists, or both, by either intracerebroventricular or intrathalamic injection, attenuated the formalin-induced pain behavior in the second phase in wild-type mice. Furthermore, activation of mGluR1 at the VPL enhanced pain behavior in the second phase in the wild-type mice. In contrast, PLCbeta4 knock-out mice did not show such enhancement, indicating that mGluR1 is connected to PLCbeta4 in the VPL. Finally, in parallel with the behavioral results, we showed in an electrophysiological study that the time course of firing discharges in VPL corresponds well to that of pain behavior in the formalin test in both wild-type and PLCbeta4 knock-out mice. These findings indicate that the thalamic mGluR1-PLCbeta4 cascade is indispensable for the formalin-induced inflammatory pain by regulating the response of VPL neurons.

Phosphorylation of extracellular signal-regulated kinases 1/2 is predominantly enhanced in the microglia of the rat spinal cord following dorsal root transection

The present study was initiated to investigate the role of extracellular signal-regulated kinases (ERK) 1/2 signaling pathway in the early response of spinal cord and associated dorsal root ganglion (DRG) to rhizotomy by using Western blotting and immunohistochemical techniques in a rat model of L3 and L4 dorsal root transection. The results showed that there were a considerable amount of total and phosphorylated ERK 1/2 protein in both spinal cord and DRG in normal animals killed under pentobarbital anesthesia. The total ERK 1/2 distributed in both glia and neurons, while phosphorylated ERK 1/2 dominantly existed in the latter in the gray matter of spinal cord, as demonstrated with double immunofluorescent staining. Twenty-four and forty-eight hours after axotomy, the phosphorylation level of ERK 1/2 in the operation side of dorsal spinal cord was much higher than that in the contralateral side, while the total ERK 1/2 level seemed unchanged. The increased expression of Fos protein was also seen in the dorsal spinal cord at lesion side twelve and twenty-four hours after axotomy. Double fluorescent staining proved that the phosphorylated ERK 1/2 positive cells in the ipsilateral dorsal spinal cord after axotomy predominantly were microglia and small portion was oligodendrocytes, whereas the Fos expression was mainly in neurons. In normal DRG, most neurons, especially the medium and small-sized ones, and the satellite cells contained total ERK 1/2-like immunoreactivity, whereas only a small portion of neurons and satellite cells contained phosphorylated ERK 1/2. After unilateral dorsal rhizotomy, there were no detectable changes for the phosphorylation of ERK 1/2 in either neurons or satellite cells in DRG.Collectively, the present results suggest that both ERK and Fos signal pathways involve the cellular activation in the spinal cord following dorsal rhizotomy, with ERK mainly in microglia and Fos in neurons. The increase of phosphorylation of ERK 1/2 in microglia of spinal cord after rhizotomy implicates that ERK signaling pathway involves intracellular activity of microglia responding to the experimental injury.

Induction of Fos protein-like immunoreactivity in the trigeminal spinal nucleus caudalis and upper cervical cord following noxious and non-noxious mechanical stimulation of the whisker pad of the rat with an inferior alveolar nerve transection

After transection of the inferior alveolar nerve (IAN: the third branch of the trigeminal nerve), the whisker pad area, which is innervated by the second branch of the trigeminal nerve, showed hypersensitivity to mechanical stimulation. Two days after IAN transection, the threshold intensity for escape behavior to mechanical stimulation of the ipsilateral whisker pad area was less than 1.0 g, a sign of allodynia, and returned to the preoperative level (preoperative threshold: 52.0 g) at 32 days after surgery. This decrement of escape threshold lasted for more than 3 weeks. The whisker pad area contralateral to the IAN transection also showed a decrease in escape threshold to non-noxious mechanical stimulation as compared with sham-operated rats. However, the change in threshold intensity for the side contralateral to transection was not as pronounced as that on the ipsilateral side. Fos protein-like immunoreactive (LI) cells were observed in the superficial laminae but not dominant in deeper laminae of the trigeminal spinal nucleus caudalis (Vc) and the first segment of the spinal cord (C1) after non-noxious mechanical stimulation of the whisker pad area in the rats with IAN transection. Fos protein-LI cells were expressed bilaterally in the Vc and C1, but were more numerous on the ipsilateral side to transection than on the contralateral side. The largest number of Fos protein-LI cells was observed at 2400 microm caudal from the trigeminal subnucleus interporalis (Vi)-Vc border both in ipsilateral and contralateral sides. The number of Fos protein-LI cells increased after application of 1, 4, and 16 g stimuli as compared to rats without mechanical stimulation. Furthermore, an extensively greater number of Fos protein-LI cells were expressed both in superficial and deep laminae of the bilateral Vc and C1 of the spinal cord after subcutaneous injection of mustard oil into the whisker pad. Fos protein expression after mustard oil injection was much stronger than that observed after any mechanical stimulation in the rats with IAN transection. These data suggest that the change in the numbers and spatial arrangement of nociceptive neurons in the Vc and C1 after IAN transection reflect the development of mechanical hyperalgesia in the area adjacent to the IAN innervated region.

Stressor specificity of central neuroendocrine responses: implications for stress-related disorders

Despite the fact that many research articles have been written about stress and stress-related diseases, no scientifically accepted definition of stress exists. Selye introduced and popularized stress as a medical and scientific idea. He did not deny the existence of stressor-specific response patterns; however, he emphasized that such responses did not constitute stress, only the shared nonspecific component. In this review we focus mainly on the similarities and differences between the neuroendocrine responses (especially the sympathoadrenal and the sympathoneuronal systems and the hypothalamo-pituitary-adrenocortical axis) among various stressors and a strategy for testing Selye's doctrine of nonspecificity. In our experiments, we used five different stressors: immobilization, hemorrhage, cold exposure, pain, or hypoglycemia. With the exception of immobilization stress, these stressors also differed in their intensities. Our results showed marked heterogeneity of neuroendocrine responses to various stressors and that each stressor has a neurochemical "signature." By examining changes of Fos immunoreactivity in various brain regions upon exposure to different stressors, we also attempted to map central stressor-specific neuroendocrine pathways. We believe the existence of stressor-specific pathways and circuits is a clear step forward in the study of the pathogenesis of stress-related disorders and their proper treatment. Finally, we define stress as a state of threatened homeostasis (physical or perceived treat to homeostasis). During stress, an adaptive compensatory specific response of the organism is activated to sustain homeostasis. The adaptive response reflects the activation of specific central circuits and is genetically and constitutionally programmed and constantly modulated by environmental factors.

Somatotopical organization of fos-like immunoreactivity in rat cervical spinal cord following noxious stimulation of the forelimb

In the present study c-fos expression has been used as a marker of neuronal activation following noxious stimuli applied to one of three different sites on the forelimb in rats. In three treatment groups (n=4 animals in each group) rats were anaesthetized with barbiturate and a mechanical pinch was applied to either (i) the most medial digit, (ii) the most lateral digit, or (iii) the shoulder area of one forelimb. An additional control group (n=4) received no pinch. The presence of Fos-like immunoreactivity was used to chart the distribution of cervical spinal cord neurons activated by the stimulus. No significant difference was found in the number of labelled cells between the contralateral side of each treatment group and either side of the control group. By contrast, there was a significant increase in labelled cells between the ipsilateral and contralateral sides within each treatment group. Labelled cells were present mainly in the dorsal horn of the ipsilateral cervical spinal cord where they were clustered in laminae I and II. Clear topographical differences were also evident between treatment groups in the distribution of labelled cells. The most medial digit was represented rostromedially compared to the most lateral digit (cell peak at segmental levels C5/C6 and C7, respectively), while the shoulder stimulus produced a more widespread distribution of labelled cells which was centred rostrolaterally (peak at segmental levels C4/C5).Overall, the findings suggest that forelimb inputs to the cervical cord are organized somatotopically in a similar fashion to hindlimb inputs to the lumbar cord, although the representation of individual forelimb digits may be more extensive in the rostrocaudal axis. This difference could reflect the use of the rats' forepaws in more complex sensorimotor tasks such as grasping and exploring objects.

Spinal sources of noxious visceral and noxious deep somatic afferent drive onto the ventrolateral periaqueductal gray of the rat

Studies utilizing the expression of Fos protein as a marker of neuronal activation have revealed that pain of deep somatic or visceral origin selectively activates the ventrolateral periaqueductal gray (vlPAG). Previous anatomical tracing studies revealed that spinal afferents to the vlPAG arose from the superficial and deep dorsal horn and nucleus of the dorsolateral funiculus at all spinal segmental levels, with approximately 50% of vlPAG-projecting spinal neurons found within the upper cervical spinal cord. This study utilized detection of Fos protein to determine the specific populations of vlPAG-projecting spinal neurons activated by noxious deep somatic or noxious visceral stimulation. Pain of cardiac or peritoneal (i.e., visceral) origin activated neurons in the superficial and deep dorsal horn and nucleus of the dorsolateral funiculus of the thoracic cord, whereas pain of hindlimb (i.e., deep somatic) origin activated neurons in the same laminar regions but in the lumbosacral cord. Each of these deep noxious manipulations also activated neurons in the superficial and deep dorsal horn and nucleus of the dorsolateral funiculus of the upper cervical spinal cord. In a second set of experiments, the combination of retrograde tracing and Fos immunohistochemistry revealed that vlPAG-projecting spinal neurons activated by deep somatic pain were located in both the upper cervical and lumbosacral cord, whereas those activated by visceral pain were restricted to the thoracic spinal cord. Thus pain arising from visceral versus deep somatic body regions influences neural activity within the vlPAG via distinct spinal pathways. The findings also highlight the potential significance of the upper cervical cord in integrating pain arising from deep structures throughout the body.

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

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

Incipient cauda equina syndrome as a model of somatovisceral pain in dogs: spinal cord structures involved as revealed by the expression of c-fos and NADPH diaphorase activity

Segmental and laminar distribution of Fos-like immunoreactive, reduced nicotinamide adenine dinucleotide phosphate diaphorase (NADPHd)-exhibiting and double-labeled (Fos-like immunoreactive and NADPHd-exhibiting) neurons was examined in lower lumbar and sacral segments of the dog spinal cord using the model of multiple cauda equina constrictions. NADPHd histochemistry was used as marker of nitric oxide synthase-containing neurons. The appearance and the time-course of Fos-like immunoreactive, NADPHd and double-labeled neurons was studied at 2 h and 8 h postconstriction characterized as the incipient phase of cauda equina syndrome. The occurrence of Fos-like immunoreactive and NADPHd-exhibiting neurons in fully developed cauda equina syndrome was studied at five days postconstriction. An increase in Fos-like immunoreactivity in superficial laminae (I-II) and an enhanced NADPHd staining of lamina VIII neurons were found. A statistically significant increase in Fos-like immunoreactive neurons was found in laminae I-II and VIII-X 8 h postconstriction, and in contrast, a prominent decrease in Fos-like immunoreactive neurons was found in laminae I-II, accompanied by a statistically significant increase in Fos-like immunoreactive neurons in more ventrally located laminae VII-X at five days postconstriction. Quantitative analysis of laminar distribution of constriction-induced NADPHd-exhibiting neurons revealed a considerable increase in these neurons in laminae VIII-IX 8 h postconstriction and a statistically highly significant increase in NADPHd-exhibiting neurons in laminae VII-X five days postconstriction. Concurrently, the number of NADPHd-exhibiting neurons in laminae I-II was greatly reduced. While a low number of double-labeled neurons was found throughout the gray matter of lower lumbar and sacral segments at 2 h postconstriction, a statistically significant number of double-labeled neurons was found in lamina X 8 h and in laminae VII-X five days postconstriction. The course and distribution of anterograde degeneration resulting five days after multiple cauda equina constrictions are compared with segmental and laminar distribution of Fos-like immunoreactive and NADPHd-exhibiting neurons. Prominent involvement of the spinal cord neurons appearing in the lumbosacral segments at the early beginning and in fully developed cauda equina syndrome results in a Fos-like immunoreactivity and strongly enhanced NADPHd staining of some neuronal pools. Under such circumstances, an early cauda equina decompression surgery is advisable aimed at decreasing or preventing the derangement of the neural circuits in the lumbosacral segments.

The role of ascending neuronal pathways in stress-induced release of noradrenaline in the hypothalamic paraventricular nucleus of rats

Central catecholaminergic pathways carrying pain-related signals to the hypothalamic paraventricular nucleus (PVN) were investigated in laboratory rats. Four per cent formalin injected subcutaneously was employed as a stressful stimulus. Neuronal activity in brainstem catecholaminergic and paraventricular neurones was assessed by Fos immunohistochemistry. Stress-induced noradrenaline (NE) release from nerve terminals in the PVN was measured in extracellular fluid by in-vivo microdialysis. Within 30 min, formalin elicited a four- to sixfold increase in plasma ACTH and corticosterone concentrations and intense Fos-like activity was seen in the superficial zones of the lumbar spinal cord ipsilateral to the side of the formalin injection. In brainstem catecholaminergic neurones, the PVN, and midline thalamic nuclei, formalin-induced Fos-immunopositivity was equally present in the ipsi- and contralateral sides of the injection. An immediate elevation (4-5 times higher than baseline levels) of NE levels was measured in both the right and left PVN after a formalin injection into the right paw. Unilateral surgical transections at the medulla-spinal cord junction failed to affect formalin-induced elevations in NE levels in the PVN independently of the side of the formalin injection or the knife cut. Thus, this observation clearly shows that fibres carrying pain-evoked signals ascend bilaterally from the spinal cord to the brainstem and forebrain. Hemisections of the medulla oblongata between the level of A1-A2 NE cell groups and the locus coeruleus reduced but did not eliminate formalin-induced NE release from the PVN ipsilateral to the knife cut. This effect was independent of the side of the formalin injection. In the contralateral PVN, high and similar NE levels were measured in response to a formalin injection into the right or the left leg. The present study indicates that formalin-induced pain signals are carried by sensory fibres to the ipsilateral spinal cord. From there, axons of different dorsal horn neurones reach noradrenergic cells on both sides of the medulla oblongata. The majority of noradrenergic fibers ascend on the same side and innervate the ipsilateral PVN. Since formalin administration resulted in a moderate elevation of NE levels in the PVN on the operated side, the role of other ascending noradrenergic (from the locus coeruleus) or noncatecholaminergic fibres that could modulate NE release from the PVN should be considered.

Interrelations of opioids with monoamines in descending inhibition of nociceptive transmission at the spinal level: an immunocytochemical study

This study was designed to reexamine a previous proposal of whether the opioid-like substances (OLS) being acting mainly as an intrinsic spinal mediator in the descending inhibition of nociception of the bulbospinally projecting NE-ergic, and/or 5-HT-ergic terminals in the dorsal horn by using an immunocytochemical method. The effects of intrathecal (i.t.) phentolamine (Ph), cyproheptadine (Cyp), and naloxone (Nal), administered separately or coadministered by two of them, on the expression of Fos-like-immunoreactive (FLI) neurons were observed on both sides of the lumbar dorsal horn of rats, in which equal volumes of formalin were injected into two hindpaws and the ipsilateral dorsolateral funiculus (DLF) was transected at the thoracic level antecedently. The results showed: (1) when rats were pretreated with i.t. saline, the number of nociceptive FLI neurons was significantly lowered 44% (p<0.01) on the side of the lumbar dorsal horn with intact DLF compared to the opposite side with sectioned DLF; (2) when rats were separately pretreated with i.t. Ph, Cyp and Nal, the reduction of FLI neurons on the DLF-intact side were decreased by 27% (p<0.01), 21% (p<0.01), and 25% (p<0.01), respectively; (3) when rats were pretreated with combined i.t. Ph+Cyp, the reduction on the intact side was eliminated almost completely (4%); (4) when rats were pretreated with combined i.t. Ph+Nal, the reduction on the intact side was 21% (p<0.01); and (5) when rats were pretreated with i.t. Cyp+Nal, the reduction on the intact side was 9.1%. These results suggest that: (1) nearly all the suppressive action exerted by the DLF-descending fibers are produced by the release of either NE or 5-HT as neurotransmitters at the spinal level; (2) most of the opioid-like substances act as an intrinsic spinal mediator mainly for the descending NE-ergic, but in a lesser extent for the 5-HT-ergic terminals in the dorsal horn circuitry; and (3) some OLS-ergic interneurons may only be activated by local nociceptive input.

Expression of c-Fos protein in the spinal cord after brachial plexus injury: comparison of root avulsion and distal nerve transection

OBJECTIVE

Clinical reports indicate poor outcomes for avulsion injuries, compared with more peripheral nerve damage. These two different injuries may both affect gene expression in spinal neurons, and the changes in gene expression may be related to the types of injuries.

METHODS

The brachial plexus of 48 adult male rats was lesioned by either root avulsion close to the spinal cord or distal nerve transection. The rats were quickly revived and remained awake until death at 30, 60, or 120 min after surgery. In rats with avulsive injuries, traumatic sites on the dorsal and ventral horns of the spinal cord were microscopically detected. Immunocytochemical analysis of the c-fos product was performed for the two experimental groups and for sham-treated control animals at the same survival times.

RESULTS

An increase in Fos-like immunoreactivity (FLI) in cells of the spinal cord, at levels C4-T1, was detected at 30 min after nerve transection or root avulsion. The number of FLI-positive cells continued to increase at 60 and 120 min after the nerve injury (P=0.001). FLI-positive cells were compared at the C7 level, in laminae 1 and 2, 3 and 4, and 5 to 10, after the two injuries and were found to be more abundant after the avulsive injury (P=0.0001); furthermore, the number of FLI-positive cells increased with time (P=0.001). In a comparison of all levels, both experimental groups demonstrated significantly greater numbers of FLI-positive cells than did controls, and the group with nerve root avulsion showed significantly (P=0.0001) more FLI-positive cells than did the group with distal nerve transection.

CONCLUSION

These results suggest that nerve root avulsion from the spinal cord leads to increased and prolonged expression of c-fos and, potentially, greatly increased transcription of new messages for recovery, survival, or cell death.

Regional changes in forebrain activation during the early and late phase of formalin nociception: analysis using cerebral blood flow in the rat

This is the first neural imaging study to use regional cerebral blood flow (rCBF) in an animal model to identify the patterns of forebrain nociceptive processing that occur during the early and late phase of the formalin test. We measured normalized rCBF increases by an autoradiographic method using the radiotracer [99mTc]exametazime. Noxious formalin consistently produced detectable, well-localized and typically bilateral increases in rCBF within multiple forebrain structures, as well as the interpeduncular nucleus (Activation Index, AI = 66) and the midbrain periaqueductal gray (AI = 20). Structures showing pain-induced changes in rCBF included several forebrain regions considered part of the limbic system. The hindlimb region of somatosensory cortex was significantly activated (AI = 31), and blood flow increases in VPL (AI = 8.7) and the medial thalamus (AI = 9.0) exhibited a tendency to be greater in the late phase as compared to the early phase of the formalin test. The spatial pattern and intensity of activation varied as a function of the time following the noxious formalin stimulus. The results highlight the important role of the limbic forebrain in the neural mechanisms of prolonged persistent pain and provide evidence for a forebrain network for pain.

Formalin-induced c-fos expression in the spinal cord of fetal rats

The maturational status of Adelta and C-fibers in the fetal rat spinal cord was examined using formalin-induced c-fos expression as a marker for neuronal activities. Awake 19-, 20-, and 21-day fetuses (FD) were injected ex utero with 5 microl of 10% formalin either into the ventral aspect of the forepaw or the hindpaw. FD 19 fetuses showed little response to the injection, but with increasing age, the fetuses exhibited more specific behaviors following injury of the paw. By FD 21, fetuses treated with formalin injection showed body curls and twitches, mouth opening, face wiping, and withdrawal of the injected paw. The anatomical data paralleled that of behavior; FD 19 animals expressed a small number of Fos labeled nuclei following the formalin injection that was not statistically different from control animals. The formalin-induced increase in Fos staining was first observed at FD 20 with a large increase in the number of Fos labeled cell occurring between FD 20 and 21. By FD 21, the pattern of Fos stained nuclei resembled that found in neonatal rats. There was constitutive bilateral staining in all untreated, saline and formalin injected fetuses that is unique to prenatal animals. Formalin treated fetuses showed constitutive level of staining in addition to the increase in the c-fos expression caused by formalin. We have thus demonstrated that, as indexed both by behavioral response and by Fos immunoreactivity, rat fetuses are capable of transmitting and responding to noxious input before birth.

Effects of intrathecal monoamine antagonists on the nociceptive c-Fos expression in a lesioned rat spinal cord

Effects of intrathecal (i.t.) administration of monoamine antagonists on formalin-induced neuronal c-Fos expression in two sides of the lumbar dorsal horn were observed in rats with unilateral transection of the dorsolateral funiculus at T11-12 level. The results showed that: 1) pretreated with i.t. normal saline (control) and then an equal volume of formalin was injected into the two hindpaws, the number of Fos-like immunoreactive neurons were 44% lower on the side of lumbar dorsal horn with intact dorsolateral funiculus (57 +/- 3.1 vs. 103 +/- 3.8). 2) Pretreatment with i.t. phentolamine (a non-selective alpha-adrenoceptor antagonist) caused an increase of Fos-like immunoreactive neurons on the intact side so showing only a reduction rate of 23% to the lesioned side (p < .01); 3) pretreatment with i.t. cyproheptadine (a 5-HT-receptor antagonist) caused a similar reduction rate of 21% (p < .01) of Fos-like immunoreactive neurons on the intact side; and 4) combined i.t. pretreatment with phentolamine and cyproheptadine caused a reduction of Fos-like immunoreactive neurons of only 4% on the intact side, namely, the differences in the number of Fos-like immunoreactive neurons on two sides of the lumbar spinal cord owing to the unilateral dorsolateral funiculus lesion were nearly abolished by i.t. coinjection of phentolamine and cyproheptadine. The results indicate that 1) peripheral noxious inputs can provoke a spinally-descending inhibitory effect on the spinal nociceptive transmission via the dorsolateral funiculus and 2) the descending fibers in dorsolateral funiculus exert their action mainly through the release of either norepinephrine or 5-HT at the spinal level.

The contribution of supraspinal, peripheral and intrinsic spinal circuits to the pattern and magnitude of Fos-like immunoreactivity in the lumbar spinal cord of the rat withdrawing from morphine

Withdrawal from morphine evokes increases in Fos-like immunoreactivity in the spinal cord, particularly in the superficial dorsal horn, laminae I/II. To determine the origin of the increased Fos-like immunoreactivity, we selectively targeted central or peripheral opioid receptors with naloxone-methiodide, an antagonist that does not cross the blood-brain barrier, or induced withdrawal after eliminating possible sources of input to the superficial dorsal horn. To induce tolerance, we implanted rats with morphine or placebo pellets (75 mg, six pellets over three days). On day 4, withdrawal was precipitated and after 1 h, the rats were killed, their spinal cords removed and 50 microm transverse sections of the spinal cord immunoreacted with a rabbit polyclonal antiserum directed against the Fos protein. In placebo-pelleted rats, none of the different procedures, viz. spinal transection, unilateral dorsal rhizotomy (L4-S2), neonatal capsaicin treatment or direct intrathecal opioid antagonist injection, induced expression of the Fos protein. However, both spinally transected and rhizotomized withdrawing animals showed significant increases in Fos-like immunoreactivity in laminae I/II, compared to intact withdrawing rats. Neonatal treatment with capsaicin, which eliminates C-fibres, did not alter Fos-like-immunoreactivity. Selective withdrawal of morphine from peripheral opioid receptors by naloxone-methiodide did not induce Fos-like immunoreactivity in the lumbar spinal cord greater than that recorded in nonwithdrawing rats. However, intrathecal injection of naloxone-methiodide increased Fos-like immunoreactivity in laminae I/II and the ventral horn to a greater extent than did subcutaneous injection of naloxone. We hypothesize that the increased Fos expression after systemic withdrawal in spinally-transected rats results from a loss of descending inhibitory control that is activated during withdrawal. The increase in withdrawal-induced Fos-like immunoreactivity after rhizotomy may be secondary to loss of inhibitory controls exerted by large diameter primary afferents or to deafferentation-induced reorganization in the dorsal horn. Since capsaicin did not alter the magnitude of Fos-like immunoreactivity in withdrawing rats, we conclude that hyperactivity of opioid receptor-laden C-fibres is not a necessary contributor to the withdrawal-induced increase in Fos-like immunoreactivity in laminae I and II. Taken together with the results recorded after intrathecal injection of naloxone-methiodide in tolerant rats, we conclude that the pattern of lumbar spinal cord Fos expression following systemic withdrawal is primarily a consequence of increased activity in opioid receptor-containing circuits intrinsic to the dorsal horn and that the magnitude of Fos expression is normally dampened by supraspinal and primary afferent-derived inhibitory inputs.

Knee joint inflammation attenuates spinal FOS expression after unilateral paw formalin injection in rat

Carrageenan-induced knee joint inflammation evoked a transient spinal FOS protein expression in neurons localized in the apical region of laminae I-III with peak activity observed 2 h after inflammation. Consistent with previously published observations, paw formalin injection evoked a distinct pattern of FOS protein expression in L3-L5 spinal segments. The majority of FOS-positive neurons were localized in the superficial dorsal horn (laminae I-II). Laminae V and VI contained moderate numbers of labeled neurons and only a few labeled nuclei were visible in laminae VII-X. In contrast, ipsilateral paw formalin injection, if administered 4 h after carrageenan-induced knee inflammation, evoked significantly fewer FOS positive neurons in all laminar and segmental levels analyzed as compared with formalin injected animals but without previous knee joint inflammation. These data indicate that primary acute or subacute nociceptive input may evoke central processes that are characterized by an inducible form of central inhibition which then may serve to modulate the subsequent spinal effect of superimposed nociceptive peripheral stimulation.

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

Rats, receiving an intrathecal pretreatment of oligodeoxynucleotide complementary to c-fos mRNA (antisense), showed no increases in Fos protein or preprodynorphin messenger RNA in the outer laminae of the lumbar spinal cord when challenged 4 h later with a 50 microliters intraplantar injection of 5% formalin. Animals pretreated with saline or sense oligodeoxynucleotide showed marked increases in Fos protein (2 h after formalin challenge) and preprodynorphin mRNA (20 h after formalin challenge) in the lumbar region of the cord ipsilateral to the side of the injection. The behavioural consequences of antisense pretreatment were an increase in the formalin-induced licking/biting responses during the tonic, but not the acute phase. These observations could be interpreted as representing a sequence of events beginning with the formalin-induced increase in the transcription factor Fos, which in turn increases the synthesis of preprodynorphin messenger RNA resulting in the production of the dynorphin opioid peptides which then exert a modulatory antinociceptive action.

The induction of Fos-like immunoreactivity by noxious thermal, mechanical and chemical stimuli in the lumbar spinal cord of infant rats

The present study examined the maturation of nociceptive primary afferents using expression of Fos-like immunoreactivity in the second-order spinal cord neurons as an anatomical and functional marker. Pinch, immersion in hot water, or formalin injection applied to the hindpaw was used as the peripheral noxious stimulus in awake 0-, 1-, 2-, 3-, and 14-day-old rat pups. On the day of birth, all 3 stimuli elicited expression of the Fos protein in dorsal horn cells indicating that nociceptive primary afferents are functional at this age. The expression of the Fos protein was related to the intensity of stimulation since greater injection volumes of formalin or prolonged application of the thermal stimulus increased the number of stained nuclei. The number of stained nuclei was age dependent and older pups exhibited a greater number of stained nuclei. The results of this study are consistent with electrophysiological studies that have demonstrated that the primary nociceptive afferents continue to mature during the rats' postnatal life. Furthermore, the number of Fos immunoreactive neurons in immature rats are age and stimulus-intensity dependent.

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 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.

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.

Diffuse noxious inhibitory controls reduce the expression of noxious stimulus-evoked Fos-like immunoreactivity in the superficial and deep laminae of the rat spinal cord

Behavioral and electrophysiological studies have shown that a noxious stimulus applied to one part of the body can reduce the response to a subsequent noxious stimulus elsewhere on the body. This phenomenon is referred to as diffuse noxious inhibitory controls (DNIC). In the present study we used immunocytochemical labeling for the Fos protein product of the c-fos proto-oncogene to determine the location of lumbar spinal nociresponsive neurons that are inhibited by a spatially remote noxious stimulus. Repetitive hindpaw pinch evoked pronounced Fos-like immunoreactivity in the superficial and deep laminae of the lumbar spinal cord. Placing the tail in 50 degrees C water before each hindpaw pinch significantly reduced Fos-like immunoreactivity in these regions. These data demonstrate that nociresponsive neurons in both the superficial and deep laminae of the spinal cord are sensitive to inhibition by a spatially remote noxious conditioning stimulus.

Expression of Fos in rat central nervous system elicited by afferent stimulation of the femoral nerve

To investigate the distribution of Fos-like immunoreactivity (FLI) in the central nervous system of urethane anesthesized rats after activation of a somatosympathetic reflex pathway, the cut central end of the right femoral nerve of 17 male Wistar rats was stimulated electrically for 1 h at parameters such that increases in heart rate and arterial pressure were elicited. Sections of brain and spinal cord were incubated in anti-Fos antibody and the presence of FLI was detected using the ABC immunoperoxidase method. In the spinal cord FLI was present in the ipsilateral lumbar spinal cord (laminae 1 and 2, 4-6 and 10) and contralateral intermediolateral nucleus in the thoracic spinal cord. In the hindbrain, FLI was present in the contralateral rostral ventrolateral medulla and bilaterally in the cochlear nucleus, external cuneate nucleus, locus coeruleus and lateral parabrachial nucleus. In the midbrain, label appeared in the Edinger-Westphal nucleus and peripeduncular nucleus on both sides. In the forebrain, FLI appeared bilaterally in the central nucleus of the amygdala, para- and periventricular hypothalamus, supraoptic nucleus, paraventricular thalamus, reuniens nucleus, subfornical organ and bed nucleus of the stria terminalis. These results define the central nervous system pathways of somatosympathetic reflexes and demonstrate that areas in the forebrain not previously known to be activated by somatosympathetic reflexes, but previously implicated in mediating the defense reaction, are activated by these reflexes.

'Mirror pain' in the formalin test: behavioral and 2-deoxyglucose studies

Subcutaneous injection of a dilute formaldehyde solution (5 or 10%) into a hind paw induced, in the majority of rats, the appearance of 'mirror pain': licking the contralateral untreated hind paw 10-60 min after injection. Contralateral licking activity was much less frequent than the ipsilaterally directed one, but the overall intensities of the two responses were positively correlated. Qualitatively, the two behaviours were similar. Functional activity levels of the lumbar spinal cord, as revealed by the 2-deoxyglucose (2-DG) technique, were increased bilaterally over the first hour after unilateral hind limb formalin injection in unanesthetized, freely moving rats. The enhancement of the [14C]2-DG uptake could be detected both in dorsal and ventral horns, as well as in the gray matter surrounding the central canal, and the anterolateral and dorsolateral funiculi. These metabolic changes may reflect an enhancement of the functional activity of both interneuronal pools and units projecting to supraspinal centers, giving rise to a referred contralateral pain.

Somatotopic and laminar organization of fos-like immunoreactivity in the medullary and upper cervical dorsal horn induced by noxious facial stimulation in the rat

The distribution of fos-like-immunoreactivity (fos-LI) in the medullary and upper cervical dorsal horn was examined following noxious facial stimulation, in order to evaluate the use of fos as a marker for neuronal activation in trigeminal nociceptive pathways. Control animals that received urethane anesthesia and no facial stimulation showed substantial bilateral labeling in the trigeminal complex that was restricted to one rostrocaudal level, at the transition between the medullary dorsal horn (nucleus caudalis) and nucleus interpolaris. Noxious mechanical stimulation (pinch) of different facial sites produced labeling in the ipsilateral dorsal horn whose distribution varied predictably with the rostrocaudal and dorsoventral position of the facial stimulation site, such that rostral facial sites were represented rostrally in the dorsal horn and dorsal sites were represented ventrolaterally. The cornea was exceptional among the facial stimulation sites in that it had a specific representation at two distinct rostrocaudal levels, in C1 and the interpolaris-caudalis transition region; the position of the rostral peak was somatotopically inappropriate, based on the representation of other facial sites. The proportion of labelling in laminae III-IV relative to laminae I-II was higher with noxious mechanical stimulation than with noxious thermal (55 degrees C) or chemical (subcutaneous injection of capsaicin) stimulation. The proportion of labelling in laminae III-IV produced by electrical stimulation of the infraorbital nerve was no greater than that produced by pinch. The results suggest that fos-LI mapping can be a useful method for the investigation of somatotopy but is subject to serious limitations when used for the investigation of laminar organization. The results also suggest that the interpolaris-caudalis transition region may have properties that are distinct from those of the rest of the trigeminal complex, possibly related to an involvement in autonomic function.