MK-801 reduces non-noxious stimulus-evoked Fos-like immunoreactivity in the spinal cord of rats with chronic constriction nerve injury

Evidence that increased cholecystokinin (CCK) in the periaqueductal gray (PAG) facilitates changes in Resident-Intruder social interactions triggered by peripheral nerve injury

Individual differences in the effects of a chronic neuropathic injury on social behaviours characterize both the human experience and pre-clinical animal models. The impacts of these changes to the well-being of the individual are often underappreciated. Earlier work from our laboratory using GeneChip® microarrays identified increased cholecystokinin (CCK) gene expression in the periaqueductal gray (PAG) of rats that showed persistent changes in social interactions during a Resident-Intruder encounter following sciatic nerve chronic constriction injury (CCI). In this study, we confirmed these gene regulation patterns using RT-PCR and identified the anatomical location of the CCK-mRNA as well as the translated CCK peptides in the midbrains of rats with a CCI. We found that rats with persistent CCI-induced changes in social behaviours had increased CCK-mRNA in neurons of the ventrolateral PAG and dorsal raphe nuclei, as well as increased CCK-8 peptide expression in terminal boutons located in the lateral and ventrolateral PAG. The functional significance of these changes was explored by microinjecting small volumes of CCK-8 into the PAG of uninjured rats and observing their Resident-Intruder social interactions. Disturbances to social interactions identical to those observed in CCI rats were evoked when injection sites were located in the rostral lateral and ventrolateral PAG. We suggest that CCI-induced changes in CCK expression in these PAG regions contributes to the disruptions to social behaviours experienced by a subset of individuals with neuropathic injury.

Neuritis and vinblastine-induced axonal transport disruption lead to signs of altered dorsal horn excitability

BACKGROUND

Many patients with neuropathic pain present without signs of nerve injury on routine clinical examination. Some of these patients may have inflamed peripheral nerves (neuritis). In this study, we have examined whether neuritis causes changes within the dorsal horn that may contribute to a central pain mechanism. Comparisons have been made to a model of axonal transport disruption induced using vinblastine, since neuritis disrupts such processes.

RESULTS

At the peak of cutaneous hypersensitivities, recordings from wide dynamic range neurons revealed increases in wind-up following neuritis but not vinblastine treatment. Ongoing activity from these neurons was unchanged. Vinblastine treatment caused a reduction in the responses of wide dynamic range neurons to noxious mechanical stimulation of the receptive field. The response of neurons to innocuous mechanical stimulation was also reduced in wide dynamic range neurons that were at a depth ≥550 µm following vinblastine treatment. An examination of the superficial dorsal horn revealed an increase in c-Fos-positive neurons in both groups following electrical stimulation of the sciatic nerve. The area of dorsal horn expressing substance P was also decreased following vinblastine treatment.

CONCLUSION

These findings indicate that a minor nerve insult, such as neuritis, can lead to changes within the dorsal horn that are consistent with a central neuropathic pain mechanism.

Intravenous neural stem cells abolish nociceptive hypersensitivity and trigger nerve regeneration in experimental neuropathy

A nonphysiological repair of the lesioned nerve leading to the formation of neurinomas, altered nerve conduction, and spontaneous firing is considered the main cause of the events underlying neuropathic pain. It was investigated whether neural stem cell (NSCs) administration could lead to a physiological nerve repair, thus to a reduction of neuropathic pain symptoms such as hyperalgesia and allodynia in a well-established model of this pain (sciatic nerve chronic constriction injury [CCI]). Moreover, since we and others showed that the peripheral nerve lesion starts a cascade of neuroinflammation-related events that may maintain and worsen the original lesion, the effect of NSCs on sciatic nerve pro- and antiinflammatory cytokines in CCI mice was investigated. NSCs injected intravenously, when the pathology was already established, induced a significant reduction in allodynia and hyperalgesia already 3 days after administration, demonstrating a therapeutic effect that lasted for at least 28 days. Responses changed with the number of administered NSCs, and the effect on hyperalgesia could be boosted by a new NSC administration. Treatment significantly decreased proinflammatory, activated antiinflammatory cytokines in the sciatic nerve, and reduced spinal cord Fos expression in laminae I-VI. Moreover, in NSC-treated animals, a reparative process and an improvement of nerve morphology is present at a later time. Since NSC effect on pain symptoms preceded nerve repair and was maintained after cells had disappeared from the lesion site, we suggest that regenerative, behavioral, and immune NSC effects are largely due to microenvironmental changes they might induce at the lesion site.

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

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

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.

Increase of 200-kDa neurofilament-immunoreactive afferents in the substantia gelatinosa in allodynic rats induced by compression of the dorsal root ganglion

STUDY DESIGN

An experimental study on mechanical allodynia, c-Fos expression, and 200-kDa-neurofilament immunoreactive (IR) afferent expression in the substantia gelatinosa related to compression of dorsal root ganglion (DRG).

OBJECTIVES

To evaluate the presence of allodynia in DRG compression model and to demonstrate that the structural changes of spinal dorsal horn related to DRG compression.

SUMMARY OF BACKGROUND DATA

A previous experimental report has demonstrated that the peripheral nerve injury may trigger some structural changes of the superficial spinal dorsal horn. These changes of the spinal dorsal horn were thought to be important for the modulation of pain sensations such as allodynia.

METHODS

Sixty-eight male rats were used. The left L5 lamina was exposed and a drill hole was made in it. A stainless rod was placed close to the left L5 DRG through the drill hole. Behavioral testing with von Frey filament was performed. On day 28 after surgery, c-Fos expression in the spinal dorsal horn by non-noxious stimulation was examined. L5 spinal cord and bilateral L5 DRG specimens were stained with antibody for 200-kDa neurofilament (RT97). In addition, 2 or 3 spinal cord sections per rats were processed for immunoelectron microscopy.

RESULTS

In the DRG compression group, the mechanical withdrawal threshold was decreased, c-Fos expression by non-noxious stimulation was observed in the spinal dorsal horn, and there were many RT97-IR afferents in the superficial spinal dorsal horn. Immunoelectron microscopic observations showed that RT97-IR terminals made synaptic contact with neurons in the superficial spinal dorsal horn. There were no significant differences in the distribution of RT97-IR neurons in DRG between compression and sham group.

CONCLUSIONS

DRG compression induced allodynia and that RT97-IR afferents increased in the superficial dorsal horn of the spinal cord. The increase of RT97-IR afferents may be related to the mechanisms for the observed allodynia.

AM404, an inhibitor of anandamide reuptake decreases Fos-immunoreactivity in the spinal cord of neuropathic rats after non-noxious stimulation

Cannabinoids like anandamide are involved in pain transmission. In this study we evaluated the effects of administrating N-(4-hydroxyphenyl)-5Z,8Z,11Z,14Z-eicosatetraenamide (AM404), an inhibitor of anandamide reuptake and monitoring the expression of c-fos, a marker of activated neurons in an experimental model of neuropathic pain (sciatic nerve tying). Fos expression was monitored 14 days after tying of sciatic nerve and 2 h after non-noxious stimulation. We showed that non-noxious stimulation increased Fos-positivity in the dorsal superficial laminae of the lumbar spinal cord of tied animals but not in the control animals. AM404 significantly reduced Fos induction in tied animals. Co-administration of cannabinoid CB1 receptor, cannabinoid CB2 receptor and transient receptor potential vanilloid type 1 (TRPV-1) antagonists reduced the effect of AM404 and this reduction was higher using cannabinoid CB1 receptor antagonist. These results suggest that AM404 could be a useful drug to reduce neuropathic pain and that cannabinoid CB1 receptor, cannabinoid CB2 receptor and vanilloid TRPV-1 receptor are involved.