Compensatory projection of primary nociceptors and c-fos induction in the spinal dorsal horn following neonatal sciatic nerve lesion

Neurexin-2 is a potential regulator of inflammatory pain in the spinal dorsal horn of rats

Chronic pain is one of the serious conditions that affect human health and remains cure still remains a serious challenge as the molecular mechanism remains largely unclear. Here, we used label-free proteomics to identify potential target proteins that regulate peripheral inflammatory pain and reveal its mechanism of action. Inflammation in peripheral tissue was induced by injecting complete Freund's adjuvant (CFA) into rat hind paw. A proteomic method was adopted to compare the spinal dorsal horn (SDH) in peripheral inflammatory pain (PIP) model rats with controls. Differential proteins were identified in SDH proteome by label-free quantification. The role of screened target proteins in the PIP was verified by small interfering RNA (siRNA). A total of 3072 and 3049 proteins were identified in CFA and normal saline (NS) groups, respectively, and 13 proteins were identified as differentially expressed in the CFA group. One of them, neurexin-2, was validated for its role in the inflammatory pain. Neurexin-2 was up-regulated in the CFA group, which was confirmed by quantitative PCR. Besides, intrathecal siRNA-mediated knock-down of neurexin-2 attenuated CFA-induced mechanical and thermal hyperalgesia and reduced the expression of SDH membrane glutamate receptors (eg mGlu receptor 1, AMPA receptor) and postsynaptic density (eg PSD-95, DLG2). These findings increased the understanding of the role of neurexin-2 in the inflammatory pain, implicating that neurexin-2 acts as a potential regulatory protein of inflammatory pain through affecting synaptic plasticity in the SDH of rats.

Transient loss of terminals from non-peptidergic nociceptive fibers in the substantia gelatinosa of spinal cord following chronic constriction injury of the sciatic nerve

It is well known that following peripheral nerve injury, there are numerous changes in neurotransmitter and neuropeptide expression in the superficial dorsal horn, the dorsal root ganglion and the periphery. Of particular interest are the relative contributions of two sub-types of unmyelinated C-fibers in the initiation and maintenance of chronic pain, the peptidergic, and the non-peptidergic. Evidence gathered in recent years has led researchers to believe that the non-peptidergic nociceptive primary afferents are functionally distinct from their peptidergic counterpart. For our study, we used a well-established animal model of constriction neuropathy (the Kruger model) and studied Wistar rats at 5, 7, 10, 15 and 21 days after nerve lesion caused by the application of a fixed-diameter polyethylene cuff to the left sciatic nerve. Animals were assessed for the onset and evolution of mechanical allodynia using calibrated von Frey filaments and were additionally tested for thermal (heat and cold) hypersensitivity. Immunocytochemical detection of calcitonin gene-related peptide (CGRP) and isolectin B4 (IB4) binding was used to visualize the dorsal horn distribution of the boutons from the peptidergic and non-peptidergic fibers respectively. Using confocal microscopy and image analysis, we detected a significant decrease in the density of IB4-labeled boutons, ipsilateral to the lesion, at seven and 10 days following nerve injury. The density of IB4-labeled varicosities retuned to control levels by 15 days. There were no significant changes in the density of CGRP-labeled varicosities at all time points examined. Applying electron microscopy, we initially detected degenerative changes in the central elements of type I glomeruli and then a considerable reduction in their number followed by recovery at 15 days post-lesion. As the central boutons of type Ia represent varicosities from the fibers which bind IB4, the ultrastructural changes confirmed that there was a bona fide transient loss of varicosities, not simply a loss of IB4 binding. These data indicate that, in this animal model, morphological changes in the nociceptive C-fiber input of the rat dorsal horn are restricted to the non-peptidergic sub-population and are transient in nature. Furthermore, such changes do not correlate with the time-course of the allodynia.

Neonatal inflammation and primary afferent terminal plasticity in the rat dorsal horn

Abnormal or excessive activity related to pain and injury in early life may alter normal synaptic development and lead to changes in somatosensory processing. The aim of the current study was to define the critical factors that determine long-term plasticity in spinal cord afferent terminals following neonatal inflammation. Hindpaw inflammation was produced in neonatal rat pups with 5 or 25 microl 2% carrageenan, and 5 or 25 microl complete Freund's adjuvant (CFA). All groups displayed a clear inflammatory response that recovered in 2 weeks in all but the 25 microl CFA group, who had persistent chronic inflammation confirmed by histological examination of the paw at 8 weeks. The 25 microl CFA group was also the only group that displayed a significant expansion of the sciatic and saphenous nerve terminal field in lamina II of the dorsal horn at 8 weeks, using wheat-germ agglutinin-horse radish peroxidase transganglionic labelling. This effect was not accompanied by changes in dorsal root ganglion (DRG) cell number, expression of activating transcription factor 3 (ATF3), or alterations in calcitonin gene related peptide (CGRP) or isolectin B4 binding; and was not mimicked by partial nerve damage. No long-term change in mechanical or thermal behavioural sensory thresholds was seen in any group. Lower dose CFA caused an acute, reversible expansion of terminal fields in lamina II in neonatal animals, while CFA did not produce this effect in adults. The duration and effect of neonatal inflammation is therefore dependent on the type and volume of inflammatory agent used. The expansion of afferent terminals in lamina II following neonatal CFA inflammation is maintained into adulthood if the inflammation is also maintained, as seen following 25 microl CFA. This effect is not seen in adult animals, emphasising the plasticity of the nervous system early in development.