Experiments on the nature of the signal that induces spinal neuroplastic changes following a peripheral lesion

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

Background

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

Results

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

Conclusions

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

Translational musculoskeletal pain research

Diagnosis and management of musculoskeletal pain is a major clinical challenge. Fundamental knowledge of nociception from deep somatic structures and related mechanisms of sensitisation have been characterised in animals but the translation into clinical sciences is still lacking. Development and refinement of mechanism-based quantitative sensory testing in healthy volunteers and pain patients have provided new opportunities to assess pain and hyperalgesic reactions. The current technologies can provide information about, for example, peripheral and central sensitisation, descending pain control, central integration and structure specific sensitisation. Such a mechanistic approach can be used for differentiated diagnosis and for target validating new and existing analgesics. Mechanistic pain assessment of new compounds under development provides opportunities for target validation in proof-of-concept studies, which generate information to be used for selecting the most optimal patients for later clinical trials. New safe and efficient compounds are highly needed in the area of musculoskeletal pain management.

Acid-sensing ion channel 3 mediates peripheral anti-hyperalgesia effects of acupuncture in mice inflammatory pain

Background

Peripheral tissue inflammation initiates hyperalgesia accompanied by tissue acidosis, nociceptor activation, and inflammation mediators. Recent studies have suggested a significantly increased expression of acid-sensing ion channel 3 (ASIC3) in both carrageenan- and complete Freund's adjuvant (CFA)-induced inflammation. This study tested the hypothesis that acupuncture is curative for mechanical hyperalgesia induced by peripheral inflammation.

Methods

Here we used mechanical stimuli to assess behavioral responses in paw and muscle inflammation induced by carrageenan or CFA. We also used immunohistochemistry staining and western blot methodology to evaluate the expression of ASIC3 in dorsal root ganglion (DRG) neurons.

Results

In comparison with the control, the inflammation group showed significant mechanical hyperalgesia with both intraplantar carrageenan and CFA-induced inflammation. Interestingly, both carrageenan- and CFA-induced hyperalgesia were accompanied by ASIC3 up-regulation in DRG neurons. Furthermore, electroacupuncture (EA) at the ST36 rescued mechanical hyperalgesia through down-regulation of ASIC3 overexpression in both carrageenan- and CFA-induced inflammation.

Conclusions

In addition, electrical stimulation at the ST36 acupoint can relieve mechanical hyperalgesia by attenuating ASIC3 overexpression.

Increased WDR spontaneous activity and receptive field size in rats following a neuropathic or inflammatory injury: implications for mechanical sensitivity

Spontaneous activity and receptive field size for spinal wide dynamic range (WDR) neurons were measured and related to the mechanical allodynia in both neuropathic (L5-L6 ligation, 14 days post-injury) and complete Freund's adjuvant-inflamed rats (CFA, 2 days post-injury). The size of the WDR receptive field located on the hindpaw expanded significantly (p<0.01) following both modes of injury, with no difference between CFA and neuropathic animals. Likewise, the spontaneous firing of WDR neurons was significantly elevated following both the CFA (4.4+/-0.6 spikes/s, p<0.01) and neuropathic (3.2+/-0.3 spikes/s, p<0.05) injuries compared to naive (2.1+/-0.2 spikes/s) and sham-neuropathic (1.9+/-0.3 spikes/s) rats. Furthermore, the spontaneous WDR activity recorded from CFA rats was also significantly greater (p<0.05) than neuropathic rats. Mechanical allodynia, as measured by application of a von Frey hair stimulus, was observed from both CFA and neuropathic rats, however, the degree of sensitivity was significantly greater (p<0.01) for the CFA animals. These data suggest that the differences in mechanical sensitivity between CFA and neuropathic rats may be related to their respective changes in WDR spontaneous activity, but not to the changes in receptive field size, and is further demonstration of the importance of spontaneous WDR activity in determining mechanical sensitivity following injury.

Effect of plantar local anesthetic injection on dorsal horn neuron activity and pain behaviors caused by incision

Hypersensitivity after tissue injury is an expression of neuronal plasticity in the central nervous system. This has been explored most extensively using in vitro preparations and animal models of inflammatory pain and chemical irritation. For pain after surgery, a similar process has been proposed. In the present study, we examined dorsal horn neuron (DHN) sensitization using the plantar incision model for post-operative pain. In behavioral experiments, the effect of a local anesthetic injection (or saline vehicle) 15 min before plantar incision on pain behaviors several days after incision was studied. Bupivacaine injection before incision prevented pain behaviors until 4 h afterwards; injection after incision produced the same effect. One day after incision, pain behaviors were not different between rats injected with saline or bupivacaine. In neurophysiologic experiments, however, bupivacaine injection blocked activation of DHNs during incision. One hour after incision, expansion of receptive fields (RFs) to pinch and increased background activity occurred in 14 of 16 neurons in the saline group but only in two of 22 neurons in the bupivacaine group. The difference was not due to a systemic effect of bupivacaine. Ten sensitized neurons were studied using the injection of bupivacaine 90 min after incision. Increased background activity (n=7) and expanded RFs (n=7) were reversed by bupivacaine. Sensitization was re-established in seven of eight neurons 2 h after injection as the local anesthetic dissipated. These results indicate that activation of DHNs during plantar incision and sensitization 1 h later are not necessary for subsequent pain behaviors. Because sensitization was reversed 90 min after plantar incision and then re-established as the local anesthetic effect diminished, enhanced responsiveness of DHN requires ongoing afferent input during the first day after incision.

Activation of spinal extracellular signaling-regulated kinase-1 and -2 by intraplantar carrageenan in rodents

We have examined the activation of spinal extracellular signaling-regulated kinase (ERK) in juvenile rats and adult mice after intraplantar carrageenan or saline and its relationship to pain behavior. In rats, intraplantar carrageenan evoked a peak five-fold activation of spinal ERK at 30 min measured by immunoblot. Saline injection resulted in a two-fold activation. This differential ERK activation correlated with a 2.5-fold greater pain response and the development of secondary hyperalgesia in carrageenan-injected rats, whereas both saline and carrageenan produced similar primary hyperalgesia. In mice, carrageenan injection produced a peak 3.5-fold activation of ERK, but saline was ineffective. We conclude that ERK activation may underlie spinal nociceptive processing and secondary hyperalgesia after carrageenan inflammation.

Establishment of bee venom-induced contralateral heat hyperalgesia in the rat is dependent upon central temporal summation of afferent input from the site of injury

The present investigation was designed to study whether central sensitization is determined by a time window of central summation of ongoing primary afferent input from a peripheral injury site. Sensitization was assessed behaviorally in the rat as contralateral heat hyperalgesia induced by injection of bee venom (BV) in the hind paw. The sciatic nerve was transected at various time points following intraplantar BV injection to analyze the time window for contralateral hyperalgesia. The results show that after a dose of 0.2 mg BV, axotomy at 5 min completely prevented contralateral heat hyperalgesia but was without effect at 10 min, whereas after a dose of 0.1 mg BV, axotomy at 10 min was able to prevent the contralateral heat hyperalgesia but remained without effect at 20 min. These findings suggest an important role of the amount of ipsilateral ongoing primary afferent in establishing the contralateral heat hyperalgesia. Moreover, by counting the total amount of paw flinches that is believed to be mediated by ongoing primary afferent input, it was shown that 87.35+/-5.36, 170.50+/-9.15 and 305.80+/-20.13 flinches were induced by 0.2 mg BV for a period of 5, 10 and 20 min, respectively. At the lower dose of 0.1 mg BV significant fewer flinches were elicited with 59.17+/-13.54, 133.00+/-22.33 and 234.00+/-36.42 within the three corresponding time windows before sciatic nerve transection. The results suggest that the amount of primary afferent input determines the time window required to establish central changes that are independent of further afferent input.

Time-course of spinal sensitization following carrageenan-induced inflammation in the young rat: a comparative electrophysiological and behavioural study in vitro and in vivo

Inflammation of peripheral tissues evokes spontaneous pain and an increased responsiveness to external stimuli known as hyperalgesia, produced by both peripheral and central changes. The central component is initiated by a sustained afferent barrage produced by sensitized peripheral nociceptors, but it is unclear to which extent ongoing nociceptive input is required to maintain these central changes. Here, we have used an isolated preparation of the spinal cord in vitro obtained from eight- to 12-day-old rats to examine spinal plasticity in the absence of naturally occurring afferent inputs. Spinal reflex responses in preparations obtained from naive rats were compared with those from animals with carrageenan-induced inflammation of one hindpaw of 3 h, 6 h and 20 h duration prior to the extraction of the cord. Measurements of thermal (heat) and mechanical hyperalgesia in awake animals were also made at the same time-points. At 6 h post-carrageenan, there was a significant increase in the wind-up evoked by trains of high-intensity (C-fibre) stimuli, and at 20 h increased responses to both trains and single high-intensity stimuli, and a novel wind-up to low-intensity (Abeta-fibre) trains were observed. In contrast, maximal behavioural hyperalgesia was observed by 3 h post-carrageenan, and thermal hyperalgesia had resolved by 20 h, although mechanical hyperalgesia remained. These results show that the induction of spinal plasticity independent of peripheral input is a progressive process with a slow time-course, since significant hyperreflexia in the isolated spinal preparation appears 6 h after inflammation and develops further within 20 h. We conclude that during the first 3 h following inflammation, hyperalgesia is the result of peripheral sensitization and of central mechanisms that depend on an ongoing peripheral input and thus changes were not observed in the isolated spinal cord.

Relationship between neuronal activity and substance P-immunoreactivity in the rat spinal cord during acute and persistent myositis

The spinal level of substance P (SP) is assumed to be an important determinant of neuronal activity under pathophysiological conditions. In rat dorsal horn neurones, impulse activity was studied during a carrageenan-induced acute (2-8 h) and a Freund's adjuvant-induced persistent (12 days) myositis and compared with the spinal substance P-immunoreactivity (SP-IR) of the same animals. Myositis-induced changes in responsiveness of the neurones reached a maximum within 2-8 h, whereas background activity of the neurones was highest after 12 days of myositis. The area of SP-IR in the superficial dorsal horn decreased during acute and persistent myositis and the integrated density of the staining was largely unchanged. The difference in time-course between neuronal activity and SP-IR suggest that during persistent myositis factors other than SP gain more influence on the behaviour of the neurones.