Sympathectomy and Sympathetic Blockade Reduce Pain Behavior Via Alpha-2 Adrenoceptor of the Dorsal Root Ganglion Neurons in a Lumbar Radiculopathy Model

Sympathetic-Sensory Coupling as a Potential Mechanism for Acupoints Sensitization

A series of studies have demonstrated acupoint sensitization, in which acupoints can be activated in combination with sensory hypersensitivity and functional plasticity during visceral disorders. However, the mechanisms of acupoint sensitization remain unclear. Neuroanatomy evidence showed nociceptors innervated in acupoints contribute to the mechanism of acupoint sensitization. Increasing studies suggested sympathetic nerve plays a key role in modulating sensory transmission by sprouting or coupling with sensory neuron/nociceptor in the peripheral, forming the functional structure of the sympathetic-sensory coupling. Notably, the sensory inputs of the disease-induced sensitized acupoint contribute to the homeostatic regulation and also involve in delivering therapeutic information under acupuncture, hence, the role of sprouted sympathetic in acupoint function should be given attention. We herein reviewed the current knowledge of sympathetic and its sprouting in pain modulation, then discussed and highlighted the potential value of sympathetic-sensory coupling in acupoint functional plasticity.

Peripheral mechanisms of chronic pain

Acutely, pain serves to protect us from potentially harmful stimuli, however damage to the somatosensory system can cause maladaptive changes in neurons leading to chronic pain. Although acute pain is fairly well controlled, chronic pain remains difficult to treat. Chronic pain is primarily a neuropathic condition, but studies examining the mechanisms underlying chronic pain are now looking beyond afferent nerve lesions and exploring new receptor targets, immune cells, and the role of the autonomic nervous system in contributing chronic pain conditions. The studies outlined in this review reveal how chronic pain is not only confined to alterations in the nervous system and presents findings on new treatment targets and for this debilitating disease.

Referred Somatic Hyperalgesia Mediates Cardiac Regulation by the Activation of Sympathetic Nerves in a Rat Model of Myocardial Ischemia

Myocardial ischemia (MI) causes somatic referred pain and sympathetic hyperactivity, and the role of sensory inputs from referred areas in cardiac function and sympathetic hyperactivity remain unclear. Here, in a rat model, we showed that MI not only led to referred mechanical hypersensitivity on the forelimbs and upper back, but also elicited sympathetic sprouting in the skin of the referred area and C8-T6 dorsal root ganglia, and increased cardiac sympathetic tone, indicating sympathetic-sensory coupling. Moreover, intensifying referred hyperalgesic inputs with noxious mechanical, thermal, and electro-stimulation (ES) of the forearm augmented sympathetic hyperactivity and regulated cardiac function, whereas deafferentation of the left brachial plexus diminished sympathoexcitation. Intradermal injection of the α₂ adrenoceptor (α₂AR) antagonist yohimbine and agonist dexmedetomidine in the forearm attenuated the cardiac adjustment by ES. Overall, these findings suggest that sensory inputs from the referred pain area contribute to cardiac functional adjustment via peripheral α₂AR-mediated sympathetic-sensory coupling.

The Characteristics and Distribution of α2D-, α2B- and α2C-Adrenoceptor Subtypes in Goats

Simple Summary

α2-Adrenergic receptors mediate many diverse biological effects of the endogenous catecholamines epinephrine and norepinephrine. Three distinct subtypes of α2-adrenergic receptors, α2B, α2C and α2D, have been identified in goats; however, the characteristics and distribution of α2-adrenoceptors in goats remain unclear. Therefore, the aim of our study was to assess the characteristics and distribution of α2-adrenoceptor subtypes in goats. Our study highlights the wide but uneven distribution of α2-adrenoceptor subtypes in goats. Additionally, our study showed that α2D-ceptor has a better analgesic effect in goats than α2B- and α2C-adrenoceptor, whereas α2C-adrenoceptor plays a more important role in thermoregulation than α2B- and α2D-adrenoceptors.

Abstract

α2-Adrenegic receptors (α2Rs) are important presynaptic modulators of central noradrenergic function (auto receptors) and postsynaptic mediators of many of the widespread effects of catecholamines and related drugs. Studies have shown that ruminants (such as goats and cattle) express special α2DR subtypes in addition to α2BR and α2CR. Real-time quantitative PCR and Western blotting were used to investigate the distribution and density of α2R in different nuclei of the goat central nervous system, selected regions of the spinal cord (L4-L6), and in various peripheral tissues. α2-AR subtype-specific antibodies were injected intrathecally and intracerebroventricularly into the tested goats to block the corresponding subtype of receptors. Pain threshold and physiological parameters were evaluated to explore the functional characteristics of α2BR, α2CR and α2DR in goats. Our results suggest that the expression of the mRNAs and proteins of all three α2R subtypes are widely but unevenly distributed in the goat CNS and peripheral tissues. Furthermore, α2DR plays a more important role in α2R-mediated analgesia in goats than α2BR and α2CR, whereas α2CR activation exerts a greater effect on body temperature than α2BR and α2DR.

Evaluation of combined radiofrequency and chemical blockade of multi-segmental lumbar sympathetic ganglia in painful diabetic peripheral neuropathy

Background

Painful diabetic peripheral neuropathy (PDPN) is one of the most common complications of diabetes. PDPN seriously affects the quality of life and is difficult to treat; therefore, there is an urgent need for new cost-effective treatment methods for PDPN.

Objective

To investigate the efficacy and safety of radiofrequency thermocoagulation (RF) combined with anhydrous ethanol (AE) chemical blockade of lumbar sympathetic ganglia (LSG) in patients with PDPN using computed tomography (CT).

Study design

Retrospective comparative study.

Setting

Shengjing Hospital of China Medical University.

Methods

Ninety patients diagnosed with PDPN were enrolled in this study. The patients were randomly divided into AE group (A, n=30), RF group (B, n=30), and RF+ AE group (C, n=30). The follow-up included preoperative basic conditions, visual analog scale (VAS), the total remission rate (TRR), skin temperature (ST) and the improvement of numbness and hyperalgesia in the lower extremities, complications, and degree of satisfaction (DOS) before and after surgery.

Results

Postoperative VASs were significantly decreased compared to preoperative VASs in all groups (P<0.05). The VAS in group A began to increase 3 months (3M) after surgery; VAS scores at 3M, 6 months (6M) and 1 year (1Y) were significantly different compared to group B and C (P<0.05); VAS in group B began to increase after 6M; VAS scores at 6M and 1Y were significantly different compared to group C (P<0.05); Moreover, group C maintained relatively long duration of pain relief. TRR in group A, group B and group C at 1Y after operation was 66.7%, 73.3% and 93.3%, respectively; TRR in group C was statistically different compared to groups A and B (P<0.05). Higher ST in the lower extremities was observed after surgery in all groups compared to peroration (P<0.05); nonetheless, the difference was not statistically significant. The numbness and hyperalgesia improved in all three groups after surgery compared to preoperational time, the numbness in group C was significantly higher compared to groups A and B. In addition, no severe complications were observed. At 6M and 1Y after surgery, the degree of satisfaction in patients from group C was significantly higher compared to groups A and B.

Conclusion

Radiofrequency thermocoagulation combined with AE chemical blockade of the LSG was safe and effective. Nevertheless, the details underlying analgesic mechanisms still need to be investigated.

Suppression of Sympathetic Nerve Sprouting by Local Administration of an α-antagonist Around the Dorsal Root Ganglion in a Lumbar Radiculopathy Model

STUDY DESIGN

Animal experimental study with intervention.

OBJECTIVE

The purpose of this study was to elucidate whether local administration of an α-antagonist around the dorsal root ganglion (DRG) suppressed sympathetic nerve sprouting, from the acute to the chronic pain development phase, in a lumbar radiculopathy model using immunohistochemical methods.

SUMMARY OF BACKGROUND DATA

The abnormal sympathetic-somatosensory interaction may underlie some forms of neuropathic pain. There were several reports suggesting α-antagonists are effective to treat neuropathic pain. However, its pathophysiological mechanisms remain obscure.

METHODS

We used 70 male Sprague-Dawley rats. After root constriction (RC), rats received a series of three local injections of the nonselective α-antagonist phentolamine around the DRG for 3 days. There were three groups of rats: those that were injected from the day of surgery and those injected from day 4 and third group injected from day 11. The control rats were subjected to RC but equal-volume normal saline injections, and the naïve rats were not subjected to any surgical procedures. At the 14th postoperative day, the left L5 DRG was removed, embedded in paraffin, and sectioned. Sections were then immunostained with antibodies to tyrosine hydroxylase (TH). To quantify the extent of the presence of sympathetic nerve fibers, we counted TH-immunoreactive fibers in the DRG using a light microscope equipped with a micrometer graticule. We counted the squares of the graticule, which contained TH-immunoreactive fibers for each of five randomly selected sections of the DRG.

RESULTS

In the naïve group, TH-immunoreactive fibers were scarce in the DRG. α-antagonist injections from postoperative day 0 and 4 suppressed sympathetic nerve sprouting compared with the control group. α-antagonist injections from postoperative day 11 had no suppressant effect compared with the control group.

CONCLUSION

The α-antagonist administered around the DRG could suppress neural plastic changes in the early phase after nerve injury.

LEVEL OF EVIDENCE

N/A.

Localized Sympathectomy Reduces Mechanical Hypersensitivity by Restoring Normal Immune Homeostasis in Rat Models of Inflammatory Pain

Some forms of chronic pain are maintained or enhanced by activity in the sympathetic nervous system (SNS), but attempts to model this have yielded conflicting findings. The SNS has both pro- and anti-inflammatory effects on immunity, confounding the interpretation of experiments using global sympathectomy methods. We performed a "microsympathectomy" by cutting the ipsilateral gray rami where they entered the spinal nerves near the L4 and L5 DRG. This led to profound sustained reductions in pain behaviors induced by local DRG inflammation (a rat model of low back pain) and by a peripheral paw inflammation model. Effects of microsympathectomy were evident within one day, making it unlikely that blocking sympathetic sprouting in the local DRGs or hindpaw was the sole mechanism. Prior microsympathectomy greatly reduced hyperexcitability of sensory neurons induced by local DRG inflammation observed 4 d later. Microsympathectomy reduced local inflammation and macrophage density in the affected tissues (as indicated by paw swelling and histochemical staining). Cytokine profiling in locally inflamed DRG showed increases in pro-inflammatory Type 1 cytokines and decreases in the Type 2 cytokines present at baseline, changes that were mitigated by microsympathectomy. Microsympathectomy was also effective in reducing established pain behaviors in the local DRG inflammation model. We conclude that the effect of sympathetic fibers in the L4/L5 gray rami in these models is pro-inflammatory. This raises the possibility that therapeutic interventions targeting gray rami might be useful in some chronic inflammatory pain conditions.

SIGNIFICANCE STATEMENT

Sympathetic blockade is used for many pain conditions, but preclinical studies show both pro- and anti-nociceptive effects. The sympathetic nervous system also has both pro- and anti-inflammatory effects on immune tissues and cells. We examined effects of a very localized sympathectomy. By cutting the gray rami to the spinal nerves near the lumbar sensory ganglia, we avoided widespread sympathetic denervation. This procedure profoundly reduced mechanical pain behaviors induced by a back pain model and a model of peripheral inflammatory pain. One possible mechanism was reduction of inflammation in the sympathetically denervated regions. This raises the possibility that therapeutic interventions targeting gray rami might be useful in some inflammatory conditions.