Behavioral and Electrophysiological Evidence for the Differential Functions of TRPV1 at Early and Late Stages of Chronic Inflammatory Nociception in Rats

Understanding of Spinal Wide Dynamic Range Neurons and Their Modulation on Pathological Pain

The spinal dorsal horn (SDH) transmits sensory information from the periphery to the brain. Wide dynamic range (WDR) neurons within this relay site play a critical role in modulating and integrating peripheral sensory inputs, as well as the process of central sensitization during pathological pain. This group of spinal multi-receptive neurons has attracted considerable attention in pain research due to their capabilities for encoding the location and intensity of nociception. Meanwhile, transmission, processing, and modulation of incoming afferent information in WDR neurons also establish the underlying basis for investigating the integration of acupuncture and pain signals. This review aims to provide a comprehensive examination of the distinctive features of WDR neurons and their involvement in pain. Specifically, we will examine the regulation of diverse supraspinal nuclei on these neurons and analyze their potential in elucidating the mechanisms of acupuncture analgesia.

Antinociceptive, anti-inflammatory and antioxidant effects of Boerhavia coccinea extracts and fractions on acute and persistent inflammatory pain models

Background Boerhavia coccinea (Nyctaginaceae) is an herbaceous plant used for the treatment of pain. The aim of this study was to evaluate the antinociceptive and anti-inflammatory activities of the aqueous (AEBC) and ethanol (EEBC) extracts of Boerhavia coccinea as well as the major fractions (F1, F2 and F3) from EEBC. Methods The antinociceptive effect of the extracts and fractions was evaluated using formalin test. AEBC, EEBC and F1 were selected and further evaluated acutely (24 h) and chronically (16 days) in Complete Freund's Adjuvant (CFA)-induced persistent inflammatory pain for their antihyperalgesic and anti-inflammatory effects. They were administered orally (100 and 200 mg/kg/day) from 48 h following the intraplantar injection of 100 µL of CFA. After the 16 days of chronic treatment, rats' spinal cord and brain were collected for the evaluation of oxidative stress parameters namely nitric oxide (NO), malondialdehyde (MDA), superoxide dismutase (SOD) and catalase (CAT). Results AEBC, EEBC and F1 significantly inhibited the first and second phases of the formalin-induced pain. They significantly reduced the hyperalgesia both in acute and chronic treatments. These extracts showed no acute anti-inflammatory effect. AEBC and EEBC exhibited anti-inflammatory activities after repeated administration. AEBC, EEBC and F1 significantly reduced MDA level and significantly increased SOD and catalase activities, mainly in the spinal cord. AEBC and EEBC also reduced the NO production in the spinal cord. Conclusions Boerhavia coccinea extracts and F1 possess potent antinociceptive activity which is not related to their anti-inflammatory properties. Their antioxidant effects may contribute to these activities in chronic treatment.

Characterization and pharmacological modulation of noci-responsive deep dorsal horn neurons across diverse rat models of pathological pain

This overview compares the activity of wide dynamic range (WDR) and nociceptive specific (NS) neurons located in the deep dorsal horn across different rat models of pathological pain and following modulation by diverse pharmacology. The data were collected by our group under the same experimental conditions over numerous studies to facilitate comparison. Spontaneous firing of WDR neurons was significantly elevated (>3.7 Hz) in models of neuropathic, inflammation, and osteoarthritic pain compared with naive animals (1.9 Hz) but was very low (<0.5 Hz) and remained unchanged in NS neurons. WDR responses to low-intensity mechanical stimulation were elevated in neuropathic and inflammation models. WDR responses to high-intensity stimuli were enhanced in inflammatory (heat) and osteoarthritis (mechanical) models. NS responses to high-intensity stimulation did not change relative to control in any model examined. Several therapeutic agents reduced both evoked and spontaneous firing of WDR neurons (e.g., TRPV1, TRPV3, Nav1.7, Nav1.8, P2X7, P2X3, H3), other targets affected neither evoked nor spontaneous firing of WDR neurons (e.g., H4, TRPM8, KCNQ2/3), and some only modulated evoked (e.g, ASIC1a, Cav3.2) whereas others decreased evoked but affected spontaneous activity only in specific models (e.g., TRPA1, CB2). Spontaneous firing of WDR neurons was not altered by any peripherally restricted compound or by direct administration of compounds to peripheral sites, although the same compounds decreased evoked activity. Compounds acting centrally were effective against this endpoint. The diversity of incoming/modulating inputs to the deep dorsal horn positions this group of neurons as an important intersection within the pain system to validate novel therapeutics.

NEW & NOTEWORTHY Data from multiple individual experiments were combined to show firing properties of wide dynamic range and nociceptive specific spinal dorsal horn neurons across varied pathological pain models. This high-powered analysis describes the sensitization following different forms of injury. Effects of diverse pharmacology on these neurons is also summarized from published and unpublished data all recorded under the same conditions to facilitate comparison. This comprehensive overview describes the function and utility of these neurons.

Electroacupuncture at Hua Tuo Jia Ji Acupoints Reduced Neuropathic Pain and Increased GABAA Receptors in Rat Spinal Cord

Chronic constriction injury- (CCI-) induced neuropathic pain is the most similar model to hyperalgesia in clinical observation. Neuropathic pain is a neuronal dysfunction in the somatosensory system that may lead to spontaneous pain. In this study, electroacupuncture (EA) was applied at bilateral L4 and L6 of Hua Tuo Jia Ji points (EX-B2) for relieving neuropathic pain in rats. Eighteen Sprague-Dawley rats were randomly assigned to three groups: sham, 2-Hz EA, and 15-Hz EA groups. Following von Frey and cold plate tests, both the 2- and the 15-Hz EA groups had significantly lower mechanical and thermal hyperalgesia than the sham group. Western blot analysis results showed that γ-aminobutyric acid A (GABA_A), adenosine A1 receptor (A1R), transient receptor potential cation channel subfamily V member 1 (TRPV1), TRPV4, and metabotropic glutamate receptor 3 (mGluR3) were similar in the dorsal root ganglion of all three groups. Furthermore, levels of GABA_A receptors were higher in the spinal cord of rats in the 2- and 15-Hz EA groups compared with the sham control group. This was not observed for A1R, TRPV1, TRPV4, or mGluR3 receptors. In addition, all the aforementioned receptors were unchanged in the somatosensory cortex of the study rats, suggesting a central spinal effect. The study results provide evidence to support the clinical use of EA for specifically alleviating neuropathic pain.

Effects of voluntary exercise on antiretroviral therapy-induced neuropathic pain in mice

Antiretroviral therapy (ART) often results in painful peripheral neuropathy. Given that voluntary exercise has been shown to be beneficial in terms of modulating pain-like behaviors in various animal models of peripheral neuropathy, we have investigated the effects of voluntary wheel running on neuropathic pain induced by chronic ART. We first established an animal model of peripheral neuropathy induced by chronic 2',3'-dideoxycytidine (ddC) treatment. We showed that mice receiving ddC (3 mg/kg/day) had increased mechanical and thermal sensitivity at 9 weeks after the onset of the treatment. We also found that voluntary wheel running attenuated or delayed the onset of ddC-induced peripheral neuropathy. This phenomenon was associated with the attenuation of dorsal root ganglion nociceptive neuron membrane excitability and reduction in the expression of the transient receptor potential cation channel subfamily V member 1 (TRPV1). Taken together, these results suggest that voluntary exercise is an effective strategy by which ART-induced peripheral neuropathy can be alleviated.

TRPV1 channel inhibition contributes to the antinociceptive effects of Croton macrostachyus extract in mice

Background

Previous study showed that extracts from Croton macrostachyus (Euphorbiaceae) exhibit analgesic effects in acute pain models. The present study evaluates the antinociceptive properties of the methanol/methylene chloride extract (MECM) of the stem bark of this plant using mice models of persistent inflammatory and neuropathic pain, and assesses its mechanism of action.

Methods

MECM was tested on Complete Freund adjuvant (CFA)-induced persistent thermal and mechanical pain, neuropathic pain induced by partial sciatic nerve ligation (PSNL), prostaglandin E₂ (PGE₂)-induced acute mechanical hyperalgesia, as well as on nociception induced by capsaicin in mice. Mechanical hyperalgesia was assessed using von Frey hair in awake mice. The mechanism of action of MECM was evaluated by using glibenclamide on PGE₂-induced hyperalgesia or rimonabant on capsaicin-induced pain.

Results

MECM administered orally at the doses of 250 and 500 mg/kg, induced long lasting and significant antihyperalgesic effects on CFA-inflammatory and PSNL-induced neuropathic pain. MECM significantly reduced the mechanical hyperalgesia induced by PGE₂ either when administered preventively or therapeutically. MECM also significantly and time dependently inhibited the capsaicin-induced nociception. These effects were not affected by glibenclamide or by rimonabant.

Conclusions

The present results demonstrate that the oral administration of MECM to mice resulted in long lasting antihyperalgesic activity in inflammatory and neuropathic pain as well as in acute and persistent pain. The mechanism underlying the long lasting MECM antihyperalgesic effect is currently unknown, but might be mediated, at least partially, through the modulation of TRPV1 receptors.

Development of heat hyperalgesia and changes of TRPV1 and NGF expression in rat dorsal root ganglion following joint immobilization

The aim of this study was to examine whether threshold to heat stimuli, and expression of transient receptor potential vanilloid1 (TRPV1) and nerve growth factor (NGF) in dorsal root ganglion (DRG) altered under conditions of long-term limb immobilization. A plastic cast was wrapped around the right limb from the forearm to the forepaw to keep wrist joint at 90° of flexion for 5 weeks. Heat hyperalgesia was tested using the plantar test at 6 h after removing cast. The rats were perfused transcardially with 4 % paraformaldehyde and DRGs were excised at 24 h after removing cast. For size distributions of the TRPV1-IR and NGF-IR neuronal profile, the DRG area measurements over 1000 DRG neurons per animal were measured in each side, on both the immobilized (ipsilateral) and contralateral sides. Ipsilateral withdrawal latency was significantly shorter than contralateral sides. Ipsilateral percentage of immunoreactive neurons in the total DRG neurons was significantly higher than contralateral sides in TRPV1-IR and NGF-IR. Long-term casting induced heat hyperalgesia, and up-regulation and phenotypic change of TRPV1-IR and NGF-IR in DRGs on the immobilized side. These DRG alterations may involve heat hyperalgesia after long-term limb immobilization.

Attenuation of TRPV1 and TRPV4 Expression and Function in Mouse Inflammatory Pain Models Using Electroacupuncture

Although pain is a major human affliction, our understanding of pain mechanisms is limited. TRPV1 (transient receptor potential vanilloid subtype 1) and TRPV4 are two crucial receptors involved in inflammatory pain, but their roles in EA- (electroacupuncture-) mediated analgesia are unknown. We injected mice with carrageenan (carra) or a complete Freund's adjuvant (CFA) to model inflammatory pain and investigated the analgesic effect of EA using animal behavior tests, immunostaining, Western blotting, and a whole-cell recording technique. The inflammatory pain model mice developed both mechanical and thermal hyperalgesia. Notably, EA at the ST36 acupoint reversed these phenomena, indicating its curative effect in inflammatory pain. The protein levels of TRPV1 and TRPV4 in DRG (dorsal root ganglion) neurons were both increased at day 4 after the initiation of inflammatory pain and were attenuated by EA, as demonstrated by immunostaining and Western blot analysis. We verified DRG electrophysiological properties to confirm that EA ameliorated peripheral nerve hyperexcitation. Our results indicated that the AP (action potential) threshold, rise time, and fall time, and the percentage and amplitude of TRPV1 and TRPV4 were altered by EA, indicating that EA has an antinociceptive role in inflammatory pain. Our results demonstrate a novel role for EA in regulating TRPV1 and TRPV4 protein expression and nerve excitation in mouse inflammatory pain models.

Chronic inflammation and estradiol interact through MAPK activation to affect TMJ nociceptive processing by trigeminal caudalis neurons

The mitogen-activated protein kinase/extracellular regulated kinase (MAPK/ERK) pathway plays a key role in mediating estrogen actions in the brain and neuronal sensitization during inflammation. Estrogen status is a risk factor in chronic temporomandibular muscle/joint (TMJ) disorders; however, the basis for this relationship is not known. The present study tested the hypothesis that estrogen status acts through the MAPK/ERK signaling pathway to alter TMJ nociceptive processing. Single TMJ-responsive neurons were recorded in laminae I-II at the spinomedullary (Vc/C(1-2)) junction in naïve ovariectomized (OvX) female rats treated for 2 days with high-dose (20 microg/day; HE2) or low-dose estradiol (2 microg/day; LE2) and after chronic inflammation of the TMJ region by complete Freund's adjuvant for 12-14 days. Intra-TMJ injection of ATP (1 mM) was used to activate Vc/C(1-2) neurons. The MAPK/ERK inhibitor (PD98059, 0.01-1 mM) was applied topically to the dorsal Vc/C(1-2) surface at the site of recording 10 min prior to each ATP stimulus. In naïve HE2 rats, low-dose PD98059 caused a maximal inhibition of ATP-evoked activity, whereas even high doses had only minor effects on units in LE2 rats. By contrast, after chronic TMJ inflammation, PD98059 produced a marked and similar dose-related inhibition of ATP-evoked activity in HE2 and LE2 rats. These results suggested that E2 status and chronic inflammation acted, at least in part, through a common MAPK/ERK-dependent signaling pathway to enhance TMJ nociceptive processing by laminae I-II neurons at the spinomedullary junction region.

The role of the TRPV1 endogenous agonist N-Oleoyldopamine in modulation of nociceptive signaling at the spinal cord level

Transient receptor potential vanilloid (TRPV1) receptors are abundant in a subpopulation of primary sensory neurons that convey nociceptive information from the periphery to the spinal cord dorsal horn. The TRPV1 receptors are expressed on both the peripheral and central branches of these dorsal root ganglion (DRG) neurons and can be activated by capsaicin, heat, low pH, and also by recently described endogenous lipids. Using patch-clamp recordings from superficial dorsal horn (DH) neurons in acute spinal cord slices, the effect of application of the endogenous TRPV1 agonist N-oleoyldopamine (OLDA) on the frequency of miniature excitatory postsynaptic currents (mEPSCs) was evaluated. A high concentration OLDA (10 microM) solution was needed to increase the mEPSC frequency, whereas low concentration OLDA (0.2 microM) did not evoke any change under control conditions. The increase was blocked by the TRPV1 antagonists SB366791 or BCTC. Application of a low concentration of OLDA evoked an increase in mEPSC frequency after activation of protein kinase C by phorbol ester (PMA) and bradykinin or in slices from animals with peripheral inflammation. Increasing the bath temperature from 24 to 34 degrees C enhanced the basal mEPSC frequency, but the magnitude of changes in the mEPSC frequency induced by OLDA administration was similar at both temperatures. Our results suggest that presumed endogenous agonists of TRPV1 receptors, like OLDA, could have a considerable impact on synaptic transmission in the spinal cord, especially when TRPV1 receptors are sensitized. Spinal TRPV1 receptors could play a pivotal role in modulation of nociceptive signaling in inflammatory pain.