Huiqing Li

Robust Increase of Cutaneous Sensitivity, Cytokine Production and Sympathetic Sprouting in Rats with Localized Inflammatory Irritation of the Spinal Ganglia

We investigated the role and mechanisms of inflammatory responses within the dorsal root ganglion (DRG) in the development of chemogenic pathological pain. DRG inflammation was induced by a single deposit of the immune activator zymosan in incomplete Freunds adjuvant in the epidural space near the L5 DRG via a small hole drilled through the transverse process. After a single zymosan injection, rats developed bilateral mechanical hyperalgesia and allodynia which began by day 1 after surgery, peaked at days 3-7, and lasted up to 28 days. The number of macrophages in ipsilateral and contralateral DRGs increased significantly, lasting over 14 days. Robust glial activation was observed in inflamed ganglia. Cytokine profile analysis using a multiplexing protein array system showed that, in normal DRG, all but interleukin (IL)-5, IL-10 and granulocyte-macrophage colony stimulating factor (GM-CSF) were detectable with concentrations of up to 180 pg/mg protein. Local inflammatory irritation selectively increased IL-1beta, IL-6, IL-18, monocyte chemoattractant protein-1 (MCP-1), and growth-related oncogene (GRO/KC) up to 17-fold, and decreased IL-2 and IL-12 (p70) up to threefold. Inflaming the DRG also remarkably increased the incidence of spontaneous activity of A- and C-fibers recorded in the dorsal root. Many of the spontaneously active A-fibers exhibited a short-bursting discharge pattern. Changes in cytokines and spontaneous activity correlated with the time course of pain behaviors, especially light stroke-evoked tactile allodynia. Finally, local inflammation induced extensive sprouting of sympathetic fibers, extending from vascular processes within the inflamed DRG. These results demonstrate the feasibility of inducing chronic localized inflammatory responses in the DRG in the absence of traumatic nerve damage, and highlight the possible contribution of several inflammatory cytokines/chemokines to the generation of spontaneous activity and development and persistence of chemogenic pathologic pain.

Systemic antiinflammatory corticosteroid reduces mechanical pain behavior, sympathetic sprouting, and elevation of proinflammatory cytokines in a rat model of neuropathic pain.

Background:Chronic pain models are commonly defined as either nerve-injury or inflammation models, but recent work suggests inflammatory processes are important in nerve injury–induced pain. Methods:In the rat spinal nerve ligation model, the authors examined effects of systemic corticosteroid triamcinolone acetonide (TA) on the cytokine protein profile and sympathetic sprouting in the axotomized sensory ganglia, excitability of sensory neurons, and mechanical sensitivity. Results:By postoperative day 3, marked increases (5- to 16-fold) in monocyte chemoattractant protein-1, growth-related oncogene (GRO/KC or CXCL1), and interleukin (IL)-6 were observed, whereas IL-4 and IL-2 levels fell more than fourfold. The increased cytokines and number of sympathetic basket formations in the sensory ganglia were reduced toward normal values by TA given starting at the time of injury. Interleukin-4 and IL-2 levels were not restored by TA. Systemic TA also reduced the firing rate and incidence of bursting activity, but not the overall incidence of spontaneous activity, in large- and medium-sized neurons. Mechanical hypersensitivity on postoperative day 3 was reduced by TA, and some effect could still be observed 4 days after cessation of TA. However, starting TA at day 7 was ineffective. Conclusions:Several components of the spinal nerve injury model are responsive to corticosteroid, suggesting inflammatory processes are important in the development of neuropathic pain. The observation that TA was effective when given starting at the time of injury suggests that steroid treatment might alter the development of chronic pain after surgical procedures that involve nerve injury, such as amputation or hernia repair.

Decreasing sympathetic sprouting in pathologic sensory ganglia: a new mechanism for treating neuropathic pain using lidocaine

&NA; Lidocaine brings relief to those suffering from certain neuropathic pain syndromes in humans and in animal models. Evidence suggests that some neuropathic pain behaviors are closely associated with extensive sprouting of noradrenergic sympathetic fibers in the dorsal root ganglia (DRG). Using immunohistochemistry, we examined lidocaines effects on abnormal sprouting of sympathetic fibers in two animal models: rats with unilateral spinal nerve ligation (SNL) and rats with complete sciatic nerve transection (CSNT). For the first time, we have demonstrated that systemic lidocaine beginning at the time of surgery via an implanted osmotic pump remarkably reduces sympathetic sprouting (2–3 fold) (e.g. the density of sympathetic fibers and the number of DRG neurons surrounded by sympathetic fibers) in axotomized DRGs in SNL rats. The effects of systemic lidocaine lasted more than 7 days after the termination of lidocaine administration. Similar results were obtained after topical application of lidocaine to the nerve trunk to block abnormal discharges originating in the neuroma in CSNT rats. Results strongly suggest that sympathetic sprouting in pathologic DRG may be associated with abnormal spontaneous activity originating in the DRG or the injured axons (e.g. neuroma). This finding provides new insight into the mechanisms underlying sympathetic sprouting and increases our current understanding of the prolonged therapeutic effects of lidocaine on neuropathic pain syndromes.

Thermosensitivity of large primary sensory neurons.

Spontaneous activity originating in the injured nerve or the dorsal root ganglion (DRG) has been implicated in the development and maintenance of neuropathic pain. The inherent characteristics of spontaneous activity and the causal factors that modulate its firing pattern and frequency are not fully understood. We attempted to assess the thermosensitivity of spontaneous activity in dorsal root ganglion (DRG) neurons in normal rats and in rats with chronic compression of the DRG (CCD) in an in vitro nerve-DRG preparation. Extracellular, dorsal root recording from 66 spontaneously active CCD Abeta fibers indicate that: (1) decreasing bath temperature from 37 to 36-26 degrees C significantly decreased the firing rate (FR) in 85% (56/66) of fibers tested, of which 19 fibers (34%) responded to temperature change at physiological range (36-37 degrees C), whereas the remaining fibers responded at lower temperature levels (26-36 degrees C); (2) cooling of the DRG increased the FR in 12% (8/66) of fibers tested; (3) similarly, the firing rate of 21/26 spontaneously active Abeta fibers from normal rats was decreased following temperature decrease; (4) intracellular recordings from 38 normal neurons revealed that cooling the DRG significantly increased the action potential (AP) threshold, AP duration, AP amplitude and afterhyperpolarization (AHP) duration, but decreased AHP amplitude, maximal depolarizing and repolarizing rates. There was no significant change in the rheobase currents or the resting membrane potential. The present study indicates that large sensory neurons with myelinated axons are temperature dependent. It also suggests that maintenance of a stable temperature is critical for reliable characterization of spontaneous activity of sensory neurons.