Reduction of Oxidative Stress as a Potential Mechanism Underlying Euphorbia bicolor Latex Extract‐Induced Analgesia in a Rat Model of Orofacial Pain
Authors: Basu Paramita , Hornung Rebecca S. , Maier Camelia , Averitt Dayna L. 
Published: June 2021

One target of pain management is the transient receptor potential V1 ion channel (TRPV1), a cation channel that is a pain generator located on nociceptors. Recent studies have reported that reactive oxygen species (ROS; oxidative stress) may cause pain through activation of TRPV1. ROS are key mediators in the development of peripheral and central sensitization in various pain etiologies, including neuropathic, inflammatory and opioid‐induced pain. Furthermore, TRPV1 can be activated and potentiated by NADPH oxidase (NOX), which plays a key role in the production of ROS. ROS can activate TRPV1 by inducing its conformational change or by increasing the plasma membrane permeability to calcium ions. In support, inhibition of ROS is effective in reducing pain. Our previous studies reported that Euphorbia bicolor (Euphorbiaceae) latex extract possesses long‐lasting peripheral analgesia, in part through TRPV1, in the inflamed hindpaw of both male and female rats. The present study hypothesized that E. bicolor latex extract‐evoked analgesia occurs via downregulation of oxidative stress biomarkers in a rat model of orofacial pain. Orofacial pain behavior was tested using the von Frey method to assess mechanical allodynia in the rat vibrissal pad. Baseline sensitivity was recorded in male and female rats followed by injections of complete Freund's adjuvant (CFA) into the left vibrissal pad to induce inflammatory pain. Mechanical allodynia was then confirmed 24 hours following CFA injections. Rats received one injection of either E. bicolor latex extract (300 μg/mg in 0.9% saline and <5% methanol) or vehicle (0.9% saline and <5% methanol) into the inflamed vibrissal pad. Mechanical sensitivity was then reassesed at 1 hour, 6 hours, 24 hours, and/or 72 hours. Data were recorded as mechanical threshold required to elicit a withdrawal response. All rats were then rapidly decapitated immediately following the last time point and trunk blood and trigeminal ganglia (TG) were collected. A separate group of rats received the same treatments and were used to collect blood and TGs at the 1‐ and 6‐hour time points. ROS in the TGs were quantified by the fluorescent dye, 2′,7′‐dichlorodihydrofluorescein diacetate method. Oxidative stress markers were measured in trunk blood by quantification of advanced oxidation protein product (AOPP) in plasma samples. ROS‐induced NOX4 protein expression was quantified by western blot analysis. Here we report that E. bicolor latex extract significantly reduced orofacial mechanical sensitivity in both male and female rats at 24 hours and 72 hours, respectively and in concurrence with our previous reports in hindpaw‐inflamed rats. Both ROS and NOX4 were significantly reduced in the trigeminal ganglia of extract‐treated as compared to vehicle‐treated rats at the same time points. In addition, plasma AOPP was also significantly reduced with the onset of analgesia. Overall, the latex extract reduced the oxidative stress biomarkers in both plasma and TGs of rats and downregulated NOX4 protein expression corresponding with the onset of analgesia. These data indicate that the extract contains phytochemicals that may serve as novel therapeutics for treating pain with an oxidative stress component.

This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.