Predictions that conduction velocities are sensitive to the distance between nodes of Ranvier in myelinated axons have implications for nervous system function during growth and repair 1–3. ...Internodal lengths defined by Schwann cells in hindlimb nerves, for example, can undergo a 4-fold increase during mouse development, and regenerated nerves have internodes that are uniformly short 4, 5. Nevertheless, the influence of internodal length on conduction speed has limited experimental support. Here, we examined this problem in mice expressing a mutant version of periaxin, a protein required for Schwann cell elongation 4. Importantly, elongation of mutant Schwann cells was retarded without significant derangements to myelination or axon caliber. In young mice with short mutant Schwann cells, nerve conduction velocity was reduced and motor function was impaired. This demonstrates a functional relationship between internodal distance and conduction speed. Moreover, as internodes lengthened during postnatal growth, conduction velocities recovered to normal values and mutant mice exhibited normal motor and sensory behavior. This restoration of function confirms a further prediction by Huxley and Stämpfli that conduction speeds should increase as internodal distances lengthen until a “flat maximum” is reached, beyond which no further gains in conduction velocity accrue 6.
► Mice expressed the Schwann cell protein periaxin lacking its N-terminal PDZ domain ► Myelination was normal, but elongation of Schwann cells was retarded ► Conduction velocities were abnormally slow but recovered as internodes lengthened ► This shows that conduction velocities in myelinated nerves reach a “flat maximum”
Laminitis is a common debilitating disease in horses that involves painful disruption of the lamellar dermo-epidermal junction within the hoof. This condition is often refractory to conventional ...anti-inflammatory analgesia and results in unremitting pain, which in severe cases requires euthanasia. The mechanisms underlying pain in laminitis were investigated using quantification of behavioural pain indicators in conjunction with histological studies of peripheral nerves innervating the hoof. Laminitic horses displayed consistently altered or abnormal behaviours such as increased forelimb lifting and an increased proportion of time spent at the back of the box compared to normal horses. Electron micrographic analysis of the digital nerve of laminitic horses showed peripheral nerve morphology to be abnormal, as well as having reduced numbers of unmyelinated (43.2%) and myelinated fibers (34.6%) compared to normal horses. Sensory nerve cell bodies innervating the hoof, in cervical, C8 dorsal root ganglia (DRG), showed an upregulated expression of the neuronal injury marker, activating transcription factor-3 (ATF3) in both large NF-200-immunopositive neurons and small neurons that were either peripherin- or IB4-positive. A significantly increased expression of neuropeptide Y (NPY) was also observed in myelinated afferent neurons. These changes are similar to those reported in other neuropathic pain states and were not observed in the C4 DRG of laminitic horses, which is not associated with innervation of the forelimb. This study provides novel evidence for a neuropathic component to the chronic pain state associated with equine laminitis, indicating that anti-neuropathic analgesic treatment may well have a role in the management of this condition.
Reactivation of latent varicella zoster virus (VZV) within sensory trigeminal and dorsal root ganglia (DRG) neurons produces shingles (zoster), often accompanied by a chronic neuropathic pain state, ...post-herpetic neuralgia (PHN). PHN persists despite latency of the virus within human sensory ganglia and is often unresponsive to current analgesic or antiviral agents. To study the basis of varicella zoster-induced pain, we have utilised a recently developed model of chronic VZV infection in rodents. Immunohistochemical analysis of DRG following VZV infection showed the presence of a viral immediate early gene protein (IE62) co-expressed with markers of A- (neurofilament-200; NF-200) and C- (peripherin) afferent sensory neurons. There was increased expression of neuropeptide Y (NPY) in neurons co-expressing NF-200. In addition, there was an increased expression of α2δ1 calcium channel, Na
v1.3 and Na
v1.8 sodium channels, the neuropeptide galanin and the nerve injury marker, Activating Transcription Factor-3 (ATF-3) as determined by Western blotting in DRG of VZV-infected rats. VZV infection induced increased behavioral reflex responsiveness to both noxious thermal and mechanical stimuli ipsilateral to injection (lasting up to 10 weeks post-infection) that is mediated by spinal NMDA receptors. These changes were reversed by systemic administration of gabapentin or the sodium channel blockers, mexiletine and lamotrigine, but not by the non-steroidal anti-inflammatory agent, diclofenac. This is the first time that the profile of VZV infection-induced phenotypic changes in DRG has been shown in rodents and reveals that this profile appears to be broadly similar (but not identical) to changes in other neuropathic pain models.
Parenterally administered lipopolysaccharide (LPS) increases the concentration of the pro‐inflammatory cytokine interleukin‐1β (IL‐1β) in the rat hippocampus and evidence suggests that this effect ...plays a significant role in inhibiting long‐term potentiation (LTP). The anti‐inflammatory cytokine IL‐10, antagonizes certain effects of IL‐1β, so if the effects of LPS are mediated through an increase in IL‐1β, it might be predicted that IL‐10 would also abrogate the effect of LPS. Here, we report that IL‐10 reversed the inhibitory effect of LPS on LTP and the data couple this with an inhibitory effect on the LPS‐induced increase in IL‐1β. LPS treatment increased hippocampal expression of IL‐1 receptor Type I protein. Consistent with the LPS‐induced increases in IL‐1β concentration and receptor expression, were downstream changes which included enhanced phosphorylation of IRAK and the stress‐activated kinases, JNK and p38; these LPS‐induced changes were reversed by IL‐10, which concurs with the idea that these events are triggered by increased activation of IL‐1RI by IL‐1β. We provide evidence which indicates that LPS treatment leads to evidence of cell death and this was reversed in hippocampus prepared from LPS‐treated rats which received IL‐10. The evidence is therefore consistent with the idea that IL‐10 acts to protect neuronal tissue from the detrimental effects induced by LPS.
Postherpetic neuralgia (PHN), a common complication of herpes zoster, which results from reactivation of varicella zoster virus, is a challenging neuropathic pain syndrome. The incidence and severity ...of herpes zoster and PHN increases with immune impairment or age and may become a greater burden both in terms of health economics and individual suffering. A clearer understanding of the underlying mechanisms of this disease and translation of preclinical outcomes to the clinic may lead to more efficacious treatment options. Here we give an overview of recent findings from preclinical models and clinical research on PHN.
Burrowing, an ethologically relevant rodent behaviour, has been proposed as a novel outcome measure to assess the global impact of pain in rats. In a prospective multicentre study using male rats ...(Wistar, Sprague-Dawley), replication of suppressed burrowing behaviour in the complete Freund adjuvant (CFA)-induced model of inflammatory pain (unilateral, 1 mg/mL in 100 µL) was evaluated in 11 studies across 8 centres. Following a standard protocol, data from participating centres were collected centrally and analysed with a restricted maximum likelihood-based mixed model for repeated measures. The total population (TP-all animals allocated to treatment; n = 249) and a selected population (SP-TP animals burrowing over 500 g at baseline; n = 200) were analysed separately, assessing the effect of excluding "poor" burrowers. Mean baseline burrowing across studies was 1113 g (95% confidence interval: 1041-1185 g) for TP and 1329 g (1271-1387 g) for SP. Burrowing was significantly suppressed in the majority of studies 24 hours (7 studies/population) and 48 hours (7 TP, 6 SP) after CFA injections. Across all centres, significantly suppressed burrowing peaked 24 hours after CFA injections, with a burrowing deficit of -374 g (-479 to -269 g) for TP and -498 g (-609 to -386 g) for SP. This unique multicentre approach first provided high-quality evidence evaluating suppressed burrowing as robust and reproducible, supporting its use as tool to infer the global effect of pain on rodents. Second, our approach provided important informative value for the use of multicentre studies in the future.
Neuropathic pain is a pervasive chronic condition that lacks adequate therapeutic treatment, making the identification of new candidate targets for drug development a priority. Underlying the ...development of this pathological pain state is a process of neuronal plasticity, termed central sensitisation that results in hyperexcitability of sensory neurons in the spinal cord. Stimulation of peripheral nociceptive inputs can cause downstream activation of kinases in the spinal dorsal horn that may contribute to the generation of this hyperexcitable state in the spinal cord. Here, using the chronic constriction injury (CCI) model of neuropathic pain, the role played by p42/44 and p38 mitogen-activated protein (MAP) kinases was addressed. Inhibition of both the p42/44 and p38 MAP kinase pathways attenuated the behavioural reflex sensitisation seen following nerve injury. The study explored the part played by spinal VPAC2 and NK2 receptors, (which respond to the afferent excitatory neuropeptides VIP and NKA respectively), in addition to glially mediated events in the activation of these kinases. Following nerve injury, both spinal activation of p42/44 and p38 MAP kinases and behavioural sensitisation (which was sensitive to p42/44 and p38 pathway inhibitors) was prevented by VPAC2, NK2 and NMDA receptor antagonists and glial or TNF-a inhibitors. The NMDA receptor, which is thought to be crucially involved in central sensitisation in the spinal cord, binds to the multivalent adapter protein PSD-95; an interaction which is necessary for the development of neuropathic behavioural reflex sensitisation. Here we show that mutant mice expressing a single point mutation in the Src homology 3 (SH3) domain of PSD-95 (PSD-95SH3W470L mutants), have intact neuropathic behavioural reflexes but blunted inflammatory responses. These findings indicate that different domains of the same protein may contribute selectively to different pain states. Examining further the role played by PSD-95, we found that the expression of both PSD- 95 and one of its signalling partner kinases, Pyk 2, was increased in the same superficial dorsal horn neurons following nerve injury. These studies suggest the importance of specific receptors and signalling pathways, non-neuronal cells and of protein:protein complexes associated with the NMDA receptor in chronic pain states and point to their future potential in the design of novel therapeutic targets.
Neuropathic pain is a pervasive chronic condition that lacks adequate therapeutic treatment, making the identification of new candidate targets for drug development a priority. Underlying the ...development of this pathological pain state is a process of neuronal plasticity, termed central sensitisation that results in hyperexcitability of sensory neurons in the spinal cord. Stimulation of peripheral nociceptive inputs can cause downstream activation of kinases in the spinal dorsal horn that may contribute to the generation of this hyperexcitable state in the spinal cord. Here, using the chronic constriction injury (CCI) model of neuropathic pain, the role played by p42/44 and p38 mitogen-activated protein (MAP) kinases was addressed. Inhibition of both the p42/44 and p38 MAP kinase pathways attenuated the behavioural reflex sensitisation seen following nerve injury. The study explored the part played by spinal VPAC2 and NK2 receptors, (which respond to the afferent excitatory neuropeptides VIP and NKA respectively), in addition to glially mediated events in the activation of these kinases. Following nerve injury, both spinal activation of p42/44 and p38 MAP kinases and behavioural sensitisation (which was sensitive to p42/44 and p38 pathway inhibitors) was prevented by VPAC2, NK2 and NMDA receptor antagonists and glial or TNF-a inhibitors. The NMDA receptor, which is thought to be crucially involved in central sensitisation in the spinal cord, binds to the multivalent adapter protein PSD-95; an interaction which is necessary for the development of neuropathic behavioural reflex sensitisation. Here we show that mutant mice expressing a single point mutation in the Src homology 3 (SH3) domain of PSD-95 (PSD-95SH3W470L mutants), have intact neuropathic behavioural reflexes but blunted inflammatory responses. These findings indicate that different domains of the same protein may contribute selectively to different pain states. Examining further the role played by PSD-95, we found that the expression of both PSD- 95 and one of its signalling partner kinases, Pyk 2, was increased in the same superficial dorsal horn neurons following nerve injury. These studies suggest the importance of specific receptors and signalling pathways, non-neuronal cells and of protein:protein complexes associated with the NMDA receptor in chronic pain states and point to their future potential in the design of novel therapeutic targets.
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