Euphorbia bicolor (Euphorbiaceae) Latex Phytochemicals Induce Long-Lasting Non-Opioid Peripheral Analgesia in a Rat Model of Inflammatory Pain

Abstract

The negative side effects of opioid-based narcotics underscore the search for alternative non-opioid bioactive compounds that act on the peripheral nervous system to avoid central nervous system-mediated side effects. The transient receptor potential V1 ion channel (TRPV1) is a peripheral pain generator activated and sensitized by heat, capsaicin, and a variety of endogenous ligands. TRPV1 contributes to peripheral sensitization and hyperalgesia, in part, via triggering the release of proinflammatory peptides, such as calcitonin gene-related peptide (CGRP), both locally and at the dorsal horn of the spinal cord. Ultrapotent exogenous TRPV1 agonists, such as resiniferatoxin identified in the latex of the exotic Euphorbia resinifera, trigger hyperalgesia followed by long lasting, peripheral analgesia. The present study reports on the analgesic properties of Euphorbia bicolor, a relative of E. resinifera, native to the Southern United States. The study hypothesized that E. bicolor latex extract induces long-lasting, non-opioid peripheral analgesia in a rat model of inflammatory pain. Both inflamed and non-inflamed adult male and female rats were injected with the methanolic extract of E. bicolor latex into the hindpaw and changes in pain behaviors were reassessed at various time points up to 4 weeks. Primary sensory neuron cultures also were treated with the latex extract or vehicle for 15 min followed by stimulation with the TRPV1 agonist capsaicin. Results showed that E. bicolor latex extract evoked significant pain behaviors in both male and female rats at 20 min post-injection and lasting around 1–2 h. At 6 h post-injection, analgesia was observed in male rats that lasted up to 4 weeks, whereas in females the onset of analgesia was delayed to 72 h post-injection. In sensory neurons, latex extract significantly reduced capsaicin-evoked CGRP release. Blocking TRPV1, but not opioid receptors, attenuated the onset of analgesia and capsaicin-induced CGRP release. Latex was analyzed by mass spectrometry and eleven candidate compounds were identified and reported here. These findings indicate that phytochemicals in the E. bicolor latex induce hyperalgesia followed by peripheral, non-opioid analgesia in both male and female rats, which occurs in part via TRPV1 and may provide novel, non-opioid peripheral analgesics that warrant further examination.

Euphorbia bicolor(Euphorbiaceae) Latex Extract Reduces Inflammatory Cytokines and Oxidative Stress in a Rat Model of Orofacial Pain

“…E . bicolor latex extract significantly reduced orofacial mechanical sensitivity in male rats ( F (1, 18) = 38.6; p ≤ 0.05) at 24 hours (Figure 2(a)) and in female rats ( F (1, 18) = 15.71; p ≤ 0.05) at 72 hours (Figure 2(b)) as compared to vehicle-treated rats and in accordance with our previous timeline of analgesia in hindpaw-inflamed rats [34]. In male rats, extract treatment significantly increased the withdrawal threshold from 0.6 g to 4.4 g, and in female rats, extract treatment significantly increased the withdrawal threshold from 0.74 g to 4.7 g. No significant differences were observed between the vehicle- and extract-treated groups at 1 and 6 hours in male and at 1, 6, and 24 hours in female rats ( p > 0.05).…”

“…In the present study, E . bicolor latex extract significantly reduced orofacial mechanical sensitivity in both male and female rats at 24 hours and 72 hours, respectively, which correspond to the same onset time of analgesia observed in our previous studies on hindpaw-inflamed male and female rats [34]. In support, a recent systematic review reported that extracts from various plants are quite effective at alleviating orofacial pain in preclinical experimental models [59].…”

“…Since it has been reported that ROS can activate TRPV1 [10, 11] and that E . bicolor evokes analgesia in part via TRPV1 [34], it is possible that E. bicolor latex extract treatment reduces AOPP, Nox4, and ROS levels leading to reduced TRPV1 activity contributing to analgesia. Thus, E. bicolor latex phytochemicals can reduce oxidative stress effects on TRPV1, thus reducing pain.…”

“…Mechanical allodynia was then reassessed at 1, 6, and 24 hours in males and 1, 6, and 72 hours in females. The latex extract concentration and time points were selected based on our previous report that the onset of extract-induced analgesia in male and female rats with hindpaw inflammation occurs at 6 and 72 hours, respectively [34].…”

“…Many Euphorbia species are known to possess phytochemicals with antioxidant and anti-inflammatory properties [26–33]. We previously reported that Euphorbia bicolor ( Euphorbiaceae ) latex extract evokes long-lasting peripheral analgesia that does not involve opioid receptors but occurs in part through TRPV1, in a rat inflammatory pain model [34]. Given that other Euphorbia species display antioxidant and anti-inflammatory properties and E. bicolor is able to induce robust analgesia, we hypothesized that E. bicolor latex extract evokes analgesia via downregulation of oxidative stress biomarkers and proinflammatory cytokines in a rat model of orofacial pain.…”